Very slow download speeds suddenly, no changes were made (upload is fine)

Location is Scottsdale, AZ 85254 if it matters. This is an incredibly frustrating problem that I've spent just about all day and yesterday evening troubleshooting, so it is very long (sorry). I work from home and need my internet working fully to do my job. I am experienced with networking, routers, modems, systems, and troubleshooting, but running out of ideas. I do have a Cox technician coming to my house tomorrow to check the physical/wiring/cabling side of things outside my house.

TLDR: Download speeds suddenly and unexpectedly dropped significantly below what my service is rated for. Extensive testing and swapping modems, routers, PC/laptops, does not point to any single failure point. Cox technicians have no idea what the problem is as everything looks healthy. Max speed was seen in a few tests, but not reproduceable. Slow speeds are consistently reproduceable.

Problem background:

I returned from a 1.5 week vacation yesterday to extremely slow download speeds. Speedtest consistently showed ~30Mbps download and 10Mbps upload. My service (at the moment) was 500Mbps down and 10Mbps up. Speeds were the same whether it was wifi or direct ethernet to my router. Before I left town I had no issues with download speeds, although the last test I have in my Speedtest log was from 5/9/2023 and it was 476Mbps down. Nothing changed with my home network.

Setup is:

- Arris SB8200 bridge mode (self purchased, not a rental)

- Asus AC3100 wifi router

- Several Windows PC's, laptops, mobile devices, Linux server, IoT, etc

Series of events and testing:

Before involving Cox support, I power cycled the modem and router half a dozen times with no changes in download speed. I also directly connected both a laptop, and a wired PC, to the Arris modem via ethernet (bypassing my router) and experienced the same slow ~30Mbps download speeds with both devices.

I logged into my Arris modem directly and the Power and SNR values were all within manufacturer spec, everything was OK and Online, it looked healthy.

I spoke with live support chat a few times, each time going through the same old rigamarole that I had already done. Power cycles, remote resets, health check on their end, etc. Same results, ~30Mbps download, 10Mbps upload, whether connected through my router or direct to the modem.

One agent said my modem was bad because he saw 30% packet loss to the modem from his side, but otherwise it showed healthy on his end. I could see no such packet loss doing my own tests. He insisted my modem was bad, that I should buy a new one, and he would upgrade me to the latest internet package of 1Gig down, 35Mbps upload (for just a couple extra bucks a month). I declined the upgrade package as I wanted to get my problem fixed first. He reset my modem one more time to test and I got disconnected from the chat.

I got another chat agent online and began the process again. While in that chat, I received a text and email that my service upgrade was confirmed, so I guess I now have 1Gig download and 35Mbps upload, even though I declined it...

Not a huge deal...except when I tested the speeds again, my download speed was still ~30Mbps. The upload speed DID update to 35Mbps upload, from 10Mbps, after yet another modem power cycle.

We went through the same old troubleshooting that I had done countless times up to this point. Again, same slow download speeds. When they asked how old my modem was, I told them ~3 years old. To which they responded "wow, you need a new modem!"....At that point I was tired of chatting with support.

I called support on the phone, hoping to get a technician dispatched. Again, went through all the same troubleshooting, only to be told they cannot send a tech out unless my service was completely down. Oh, I was told to get a new modem again and they assured me it would fix my problem (despite them saying everything looked fine on their end).

So, I went to Best Buy and bought a brand new Arris SB8200 modem.

I called Cox support again and got a technician on the phone to register my new modem. After a bunch of waiting, the modem rebooting, and provisioning seemingly many times, it got all setup and connected. I tested again and as I expected, same problem, download speeds of only ~30Mbps. This was with my laptop directly wired to the new SB8200 modem, bypassing my router. 

The technician on the phone said everything looked good on his end and he was at a loss, he forced provisioning/resets a few times and could see nothing wrong. When I logged into the modem directly, the Power/SNR values all looked nearly exactly the same as my original modem (in spec).

With the tech on the phone, I tested with my router connected to the modem, then directly to my PC again, and back to my laptop directly. All tests were the same with ~30Mbps download and 35Mbps upload.

Here is where it gets even more weird...

While on the phone with the tech, I asked him to make my original modem the active one in the system, so I could use that and return the new one I just spent $150 on. I unplugged the brand new modem and plugged my original modem back in. I had him wait until it was back online to make sure it worked and I had at least some internet connectiviy. It came back up and I still had the same slow download speeds. I was directly connected to the modem with my laptop via ethernet.

Next, he started working on scheduling a technician to come onsite as he was out of ideas (this tech was the most helpful of the 5 or 6 I had already spoken to). This took a while since he couldn't just send out a support tech apparently, since my service got changed (somewhat unintentially) and he needed to find a service installation tech (different schedules/techs?).

While he was doing this, I hit Go on my speedtest app again for the heck of it, and suddenly I had ~950Mbps download!?!? What the heck? I told him and he said he didn't do anything and had no idea why it was working. I had not changed anything either.

So, I disconnected my laptop from the modem and plugged my router into the modem to test. Power cycled the modem and router again and waited for connectivity to come back. I tested both wired and wifi machines and again it was back to ~30Mbps. Soooo, bad router? Doesn't make sense, since it has worked fine up until just recently, and only the download is broken.

He scheduled a tech to come onsite and we left the phone call.

I then got out a backup Linksys router and factory reset it, to confirm if it was a router issue. I configured it with basic settings and power cycled the modem. Router came up with a WAN IP and I had internet connectivity. Speed tests still showed only ~30Mbps download, upload was fine at 35Mbps.

I got out another backup router, Asus AC1900, factory reset it and connected it to my modem. Power cycled the modem, let router get WAN IP and connectivity. Tested again and still had slow download speeds.

I again switched back to directly connecting the modem to my laptop, via ethernet. Power cycled modem, waited for it to re-establish connectivity. I expected to see ~900Mbps download like I had just seen earlier, but no, it was back to being slow at ~30Mbps download.

Once again I called support and the initial support person said I needed to talk to the "CAG" group with internet team. They transferred me there and I explained everything that had happened and asked to get an expedited tech onsite (the original scheduled tech was over a week out). This tech ran several tests again and said everything looked fine. While on the phone with this technician, I again ran the speedtest while connected directly to the modem and the download speeds WORKED again for a 2nd time. I saw ~950Mbps download. Again, nothing was changed, but I was able to reproduce the test a few times.

I connected my original router back up and once again it went back to slow speeds, ugh. The tech was able to find an onsite window for tomorrow (Tuesday) so I took that and we dropped the call.

After that, I again reconnected my laptop directly to the modem to test again. This time, speeds were back to being slow at ~30Mbps, even directly connected, when it worked just 25 minutes prior (as seen in image below).

As I am typing this, directly connected to my modem with the laptop, I am still seeing slow speeds of ~30Mbps download. I have no idea why it worked a few times and neither do any of the Cox technicians.

If there are any ideas out there I would love to hear them. The only consistent behavior is slow speeds when the modem is connected through ANY router. But, the router connection was working fine for years up until a week or two ago. Plus, I've tested 3 different routers. Fast download speed happened mysteriously a few times when directly connected to the modem, I cannot reproduce it successfully, and most of the time it is slow ~30Mpbs or less download.

Below image are tests from my laptop. The ~900Mbps download tests only happened when directly connected to the modem. However, several of the ~30Mbps tests were also directly connected to the modem (as well as the router). I did several other tests on my PC, so not all of them are in this image.