Ultimate Internet Speed Recently took a nosedive

I'm in the Cleveland market, and I've got the Ultimate Internet package and have been pretty happy about the speeds I get usually.  About two weeks ago, I started really noticing that my transfer rates took a nosedive: I was uploading some backups to a remote server, and where I used to get around 20 - 25 Mbps, I was now getting only around 2 Mbps.  I'm sort of a geek, so I run Zabbix to monitor pretty much everything on my network, and I pulled up a graph showing upload speeds over the last six months, and I'm not crazy (I had to split the image into left and right parts due to upload image size restrictions):

The dates across the bottom are in the form "Day.Month", so "08.02" is Feb 2. 

The speeds listed are in megabytes per second, not megabits, so from the graph you can see that, up until about the beginning of February (labelled "01.02"), I was routinely getting around 3 MB/s ~ 24 Mbps.  Then, my speeds started falling quite precipitously until by mid-February, it was topping out at about half that, 1.5 MB/s or about 12 Mbps.  They recovered slightly towards the end of March, but since April 5 ("05.04"), they've been almost completely abysmal.  Now I'm getting closer to 150 KB/s ~ 1.2 Mbps.

I was wondering if maybe, since Cox increased the speed of the slower tiers of internet, this was negatively impacting the amount of bandwidth available and the network's not able to handle the traffic. 

Furthermore, the speeds are also much less predictable and more erratic, as you can see from the right side of the graph.  I have about 10 - 15 GB of backups I upload a week, and with this 50-fold decrease in speed, there's just not enough bandwidth to push those out.

I should also note that nothing has changed on my end: same router (Buffalo WHR-G300N running DD-WRT v24), same modem (DOCSIS 3 capable, Motorola SB6180 running firmware version SB6180-3.5.8.4-GA-00-507-NOSH), and all these transfers are being done over wired connections, so it's not a wi-fi issue. Also, since the graph above is showing all traffic going to the modem from the router, it's obvious that there isn't any "hidden" traffic that could be slowing down other machines' traffic.

Has anyone else experienced this issue recently? if so, what do you recommend?

Cox, are you are aware of any speed issues recently?

Finally, in order to verify this via speed tests, the speed testing sites speedof.me, testmy.net, and myspeed.visualware.com all confirm this.  On the other hand, the speedtest.net site shows an upload speed of about 12 Mbps, which makes me think that there's some funny packet shaping or routing being done by someone somewhere that ensures that Ookla's speed tests are given some sort of priority so that the ISP looks good, although I should be clear that this is a statement of personal opinion and not fact, and I am not trying to impugn the integrity or trustworthiness of any person or organization in stating this opinion.

Thanks,

Nate