MB8600: Low SNR and huge uncorrected errors on specific downstream channels, no splitters

Hi,

I am seeing an issue that looks very similar to one I had years ago on this same Motorola MB8600:
https://forums.xfinity.com/conversations/your-home-network/mb8600-millions-of-uncorrected-errors-low-signal-to-noise-ratio-snr-slow-gigabit/602dae7ac5375f08cdeb752b

In that older thread, bypassing splitters fixed the issue. This time, there are no splitters and no extra coax. The modem is connected with about 12 inches of coax coming directly from outside the house.

Current status after about 20 hours of modem uptime:

- Modem is online and locked, with Network Access allowed.
- Downstream power levels mostly look normal, generally around -2 to 0 dBmV, with the OFDM PLC around -4 dBmV.
- Upstream channels are locked and around 44.0 to 44.5 dBmV.
- The problem is concentrated on specific downstream channels with very poor SNR and huge uncorrected errors.

Worst downstream channels:

555 MHz / Channel ID 19: SNR around 29.7 dB, 23M+ uncorrected
561 MHz / Channel ID 20: SNR 0.0 dB, 164M+ uncorrected
567 MHz / Channel ID 21: SNR around 29.7 dB, 84M+ uncorrected
573 MHz / Channel ID 22: SNR around 31.6 dB, 1.5M+ uncorrected
627 MHz / Channel ID 31: about 1M uncorrected
633 MHz / Channel ID 32: about 1M uncorrected

The event log repeatedly shows CM-STATUS messages for channel IDs 19, 20, 21, 22, 31, and 32, plus repeated MDD message timeout warnings.

I also checked the modem spectrum analyzer. The full-span sweep looks uneven, with noticeable drops/notches. I can provide screenshots of the connection page, event log, and spectrum analyzer.

Given that there are no splitters and essentially no inside coax to bypass, does this point to downstream ingress/noise or a drop/tap/plant issue? Could Xfinity check the drop, fittings, tap, and downstream impairment around those frequencies?

Thanks.