Only 8/16 downstream channels bonding, uneven speeds
In early April, I upgraded my modem from an old 8x4 modem to a 16x4 Motorola MB7420. I bought a gaming system to help me get through quarantine and wanted a more stable internet connection to allow me to play consistently. I'm on the 200Mbps plan, and with the old modem I was seeing speeds around 100-120Mbps but with an unacceptable degree of latency.
The new modem connected and was working great, I was getting speeds of 120-150Mbps with no latency issues, until the last week. I started to experience intermittent service drops and periods of unresponsive DNS wherein my entire network and every connected device would go down. I checked the modem's interface and found it was only connecting to (depending on the time) between 7 and 9 downstream channels, while the upstream channels looked fine.
I contacted Comcast and they sent a tech out to investigate. The tech tested the exterior line, the ingress line into the house, and the cable at the point where it physically plugs into the modem. All 3 tests came back within spec, so he and I both assumed it was a problem with the modem, and I ordered a replacement.
The replacement arrived this afternoon, and after I provisioned it, it immediately started having the same problems. It seems unlikely that I got two dud modems in a row, but I'm at my wits' end as to what the issue is here. I'm attaching a screenshot of my modem's interface for reference. You can see that all of the channels are out of spec (the test of the cable at the point it enters the modem had all signal levels within +/-5 dBmV), channels 9-16 are not locked at all, and there's already an insanely high error rate on channel 8.