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nhluhr's avatar
New Contributor II
4 years ago

Gigablast service, works after modem reset but ~daily gets capped to 10Mbps

I recently noticed frequent (daily) drops in my internet speed and now that I've been paying attention, I want to share the symptoms and get suggestions on what may be the best solution.

It started roughly 2 months ago.  I had been getting pretty solid speed with the following network setup:  Arris SB8200 with Amplifi HD and a single mesh point.  My speed tests run via to the nearest Cox location would return in the neighborhood of 700Mbps down and 10Mbps up.  Switching to other servers on Speedtest would normally drop the down speed but sometimes give me in the neighborhood of ~30Mbps up.  Then when it started, I'd see a limit of 10Mbps up and down regardless of speedtest location.  When I try to login to the SB8200 internal page, it wouldn't open.  After resetting the modem (either by power off/on or via Cox account website) I could then again log in and speeds would be restored.  On the cable modem's event log I would see logged events like "SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=. . ."..  At first it was occasional but it has become daily or more frequent.  About 1 month ago, I switched the router to an Amplifi Alien and the symptoms have continued, except now after a reset I get 900+ Mbps download speed, until whatever is causing the issue happens and it once again drops to 10.

In the modem's internal page, the power levels are mostly 4-7dBmV with exception of Channel 159 which is showing ~10-12 dBmV.  All the SNRs are pretty solid (41-43dB).  The only other weird thing is the very high corrected count for channel 159 (hits over 10million after just a few minutes) but I've read it's normal to have a high count here.  However I wonder if mine is higher than normal and along with the higher power level on that channel, is something wrong there?  There are 0 correcteds on other channels and 0 uncorrectables on any channel.

  • Hi, I just wanted to follow up to say thank you!  A technician came today and did some testing to determine my service cable needed grounding (it was a straight run all the way from pole to modem with no grounding blocks, terminals, or junctions) and a 'pad' to bring the Tx numbers into spec.  Ever since, the connection has been totally stable and reliable.  Problem solved!

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  • @NHluhr, The modem is reporting some poor signals on your upstream channels. Are there any splitters or signal amps on the cable line going to the modem? -Allan, Cox Support Forums Moderator.
    • nhluhr's avatar
      New Contributor II

      No, the cable comes from the pole at the street to a springy looking physical attachment on the side of my house, and then runs to a penetration in the wall, sealed with silicone, and straight to the back of the modem without any splitters or junctions.  When I moved to this house ~6.5 yr ago, there were multiple junctions spreading the coax to other parts of the house but we removed all those and ran the cable direct.

      On the upstream channels, the power readings are all 29-30 dBmV but my side doesn't show much else (like SNR) for upstream.

    • nhluhr's avatar
      New Contributor II

      also seeing this error in the log now instead of the one I mentioned above:

      ""SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;CM-MAC=. . ."

      • MaryF's avatar
        Former Moderator
        Hi Nhluhr,

        We may need to set up a service appointment so that it could be further investigated. Please send us an email to with your full name, account holder's name, street address and details of your issue or a link to your forum post.

        Cox Support Forums Moderator
  • AHS's avatar
    New Contributor

    They exact same thing happens to me. I have to reset my modem to keep my speed/bandwidth that i pay for from COX. I'm in the Pan handle of Florida. Has been this way for a while now. 

    • BenS1's avatar
      Former Moderator
      Hi AHS,
      That definitely doesn't sound good. Please contact us via Private Message Facebook or DM Twitter @coxhelp or email us at with your full name and address.

      Ben S.
      Social Media Support Specialist