Illegal set operation failed

Main PC has "Network, No Internet". WiFi has "Connected, No Internet". All lights green; flashing globe. Sometimes it will work after an unplug and replug; It'll be blue... for awhile. When the internet fails, the blue light might falsely stay on, but eventually the lights will catch up with the failure. Windows 10 troubleshooting hits "no Internet access" although my network is fine - it recommends rebooting the modem.

This started in earnest Saturday 22 August 2020. Wednesday PM 26 August was bad - I had to drive into work. This morning was the worst: over four hours. My car happens to be engine-malfunctioning so I cannot drive anymore.

My ISP (Comcast) always blames the modem. Local Arris allows login : http://192.168.0.1/home.asp and then Status>Event Log. Connectivity State should be "Operational" when things go well (like now).

Saturday 29 Aug I woke up 5:15 AM PST, and noticed a cycle from that morning. Everything had been hunky dorey last night [after going through similar]. Here is the first error after bootup:

	TLV-11 - Illegal Set operation failed;CM-MAC=...;CMTS-MAC=...;
    CM-QOS=1.1;CM-VER=3.0;

Then three-four minutes later these together :

	TFTP failed - Request sent - No Response;CM-MAC=...;CMTS-MAC:...;
	CM-QOS=1.1;CM-VER=3.0;
	TFTP Request Retries exceeded, CM unable to register

Sometimes I get a "notice", Tftp Init Failed - Reinitialize MAC. Notices get purged from the log over time. The Criticals are maintained, up to a certain number. Usually about 90 minutes on a morning like this morn.

8-10s after that, I get stuff like this:

	SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing;;
	CM-MAC=..;CMTS-MAC=...;CM-QOS=1.0;CM-VER=3.0;

At 6 AM, I did that reboot again; at 6:05 also got

	No Maintenance Broadcasts for Ranging opportunities received - T2 time-out;
	CM-MAC=...;CMTS-MAC:...;CM-QOS=1.1;CM-VER=3.0;

then

	Started Unicast Maintenance Ranging - No Response received - T3 time-out;
    CM-MAC=...;CMTS-MAC:...;CM-QOS=1.1;CM-VER=3.0;

It does this right after that : Honoring MDD; IP provisioning mode = IPv6. That "MDD Notice" is associated with a manual restart and, this morning, Illegal Set again.

Anyway, since I'm on the Arris system, I doubt the problem is in my modem and it is certainly not in this PC. There are apparently laws about installing firmware - Xfinity does it, we don't. Spectrum and Xfinity / Comcast both think it is "noise" in my cables. It may be that the cables are faltering in this house. It may be the cables need to be shielded from other electronics.

I notice that it is always the end of August (since 2005ish) that I get connection problems. School's in?