CM400: Post-Reset Fast Speeds, then Slow (Downstream Channel Shift?)

[brabq]

I have a CM400 (Netgear) cablemodem, which has worked fine for the last several years. In the last 24 hours, however, I've seen it slow to a crawl. Resetting the cable modem - whether I do it, or Xfinity does it remotely - results in fast speeds for pretty much exactly 13 minutes, at which point it then slows to a crawl.

Looking at the CM400 modem status, I can see that the "Acquired Downstream Channel" shifts in frequency when this occurs, right at 13 minutes post-reset. Here's the summary:

Immediately after reboot:

Downstream Channel 723000000 Hz Locked, QAM 256, Channel ID 41, Power 4.3 dBmV, SNR 38.6 dB

Speeds as measured with web-based speed tests @ ~100 Mbps down.

10-15 minutes after reboot:

Downstream Channel 663000000 Hz Locked, QAM 256, Channel ID 31, Power 4.7 dBmV, SNR 38.2 dB

Speeds as measured with web-based speed tests @ ~2 Mbps down.

Any ideas what may be going on here? Thanks!

[brabq · Accepted solution]

For what it's worth, and for anyone that should find this in the future.  Oddly enough, disconnecting the CM400 from power for some time (minutes) before reconnecting it seems to have solved the issue.  Now, the uptime has been >60 minutes and the modem has continued to use the initial/boot downstream channel and has not "chased" the apparently bad-choice downstream channel as it previously had.  I hope this helps someone else.

[FURRYe38]

Please post a copy and paste of the modems connection status page.
Have the ISP check the signal and line quality UP to the modem. Be sure the ISP provisions the modem correctly.
Be sure there are no coax cable line splitters in the between the modem and ISP service box. 
Be sure your using good quality RG6 coax cable up to the modem. 
Be sure to power OFF the modem for 1 minute them back ON.
https://community.netgear.com/t5/Cable-Modems-Routers/General-info-and-Troubleshooting-for-Cable-Mod...

[brabq]

FURRYe38:

Thanks.  Here's what the page looks like currently.  I have performed several resets, and - like clockwork - at "uptime" of 13m0s is when it switches downstream channels and the quality drops.

<tabindex=-1>Startup Procedure</tabindex=-1> Procedure Status Comment Acquire Downstream Channel 663000000 Hz Locked Connectivity State Ok Operational Boot State Ok Operational Security Enabled BPI+ IP Provisioning Mode Honor MDD honorMdd(4) Downstream Bonded Channels Channel Lock Status Modulation Channel ID Frequency Power SNR Correctables UnCorrectables 1 Locked QAM 256 25 627000000 Hz 4.8 dBmV 37.9 dB 7 2104 2 Locked QAM 256 26 633000000 Hz 4.7 dBmV 38.2 dB 44 2963 3 Locked QAM 256 27 639000000 Hz 4.7 dBmV 38.2 dB 16 2032 4 Locked QAM 256 28 645000000 Hz 4.7 dBmV 38.6 dB 21 1299 5 Locked QAM 256 29 651000000 Hz 4.3 dBmV 38.2 dB 29 2214 6 Locked QAM 256 42 657000000 Hz 2.2 dBmV 36.3 dB 25 3867 7 Locked QAM 256 31 663000000 Hz 4.0 dBmV 38.2 dB 29 2102 8 Locked QAM 256 32 669000000 Hz 4.0 dBmV 38.2 dB 58 3001 Upstream Bonded Channels Channel Lock Status US Channel Type Channel ID Symbol Rate Frequency Power 1 Locked ATDMA 1 5120 Ksym/sec 16400000 Hz 39.0 dBmV 2 Locked ATDMA 2 5120 Ksym/sec 22800000 Hz 39.0 dBmV 3 Locked ATDMA 3 5120 Ksym/sec 29200000 Hz 39.0 dBmV 4 Locked ATDMA 4 5120 Ksym/sec 35600000 Hz 39.0 dBmV Current System Time: Mon Feb 8 13:37:31 2021 System Up Time: 02:09:57

[brabq · Accepted solution]

For what it's worth, and for anyone that should find this in the future.  Oddly enough, disconnecting the CM400 from power for some time (minutes) before reconnecting it seems to have solved the issue.  Now, the uptime has been >60 minutes and the modem has continued to use the initial/boot downstream channel and has not "chased" the apparently bad-choice downstream channel as it previously had.  I hope this helps someone else.