57dBmV upstream

Could they be checked? Yes. Can anyone here check them? No. This is a user forum. If you are looking for a technician, either call Cox or email them at  cox.help@cox.com 

Sounds like noise on the line which is making Cox's CMTS force the modem to broadcast louder. When the noise comes, your upstream goes up, when noise goes away, your upstream goes down. Have you checked and bypassed any wiring inside your house that could be the issue?

This was interesting.  I went to where it gets distributed into the house, pulled the line that had been going to the cable modem, and attached the cable modem right off the CMCDT2106T digital tap OUT, still 57db.  Then I went to the very top of the chain in the house, a ground block.  It was 33dB before the ground block, 37 after the splitter, and now 41 at the same CMCDT2106T OUT port that read 57 a few minutes earlier.  I reconnected the cable to the house wiring and the signal at the cable modem in its normal location was still 41.  I have come across something similar in the past where disconnecting and reconnecting at the ground block seemed to reset to a good level.  I went away for a few hours, and see it is back to 57 now, the log shows this around an hour after I reconnected the ground block: Started Unicast Maintenance Ranging - No Response received - T3 time-out.

So it looks like disconnecting from the house network and reconnecting resets it to a good level... for a little while anyway?

Sounds like EMI ingress is maybe building up in the wires, lowering your US SNR? MoCA or have any strong broadcasters (IE HAM)

What does that tap do exactly? I found this forum post but I am still not 100% sure. Could a moderator give input? Doesn't look like a normal pole tap but it's not a splitter either. I read it loses about 3dB downstream, but I can find any mention of upstream gain. 

It is also possible the upstream was just fluxiating that much as you troubleshoot. After troubleshooting, did you notice the upstream still changing?

Last, how is your coaxial grounded? 

The technician came out yesterday and found a few issues.  He was able to make the line stable, with good signal strength by putting in a short, smaller cable where it comes out of the distribution box.  Before that, the thicker cable was sharply bent down and apparently causing a line transmission issue.  Noise was also found on one of my lines, and tamed by re-cabling some items and replacing an old amp.  Also, a couple of the CMCDT2106TC-A taps were replaced with splitters.  So far the readings on my line look very good, getting no uncorrectables showing for 14 of the 16 channels on my SB6183 after a day (10 and 11 still have many, but earlier this was the case for all the channels).  Upstream channels so far are staying at around 41dBmV. 

Glad I had it done, hope this message doesn't jinx it.

Yea, had a feeling it was the taps. Even knowing the model number I am still not 100% sure what they do. Not on Antronix's site.