Modem losing DOCSIS registration several times daily - T3/T4 ranging timeouts, levels in spec

  I'm getting repeated internet dropouts caused by my cable modem losing DOCSIS
  registration and re-registering. Each event drops the connection for roughly
  30 seconds. It's happening multiple times a day and appears to be getting
  more frequent.

  Modem: Arris SB8200 (DOCSIS 3.1), in bridge mode behind my own router.

  Recent events (local time):
    Aug 11  11:16 PM
    Aug 12  12:39 PM

    Aug 13   2:30 PM, 3:31 PM, 4:34 PM, 4:44 PM, 5:01 PM, 7:34 PM
    Aug 14   1:08 AM, 8:42 AM

  Signal levels are in spec in BOTH directions, which is why I don't think this
  is a levels problem:

    Downstream (32x QAM256, 399-585 MHz):  7.0 to 7.6 dBmV, SNR 44.6-45.3 dB
    Downstream uncorrectables:              0 on all 32 channels
    Upstream (4x SC-QAM, 16.4-35.6 MHz):    43-44 dBmV
    OFDM 690 MHz:                           9.1 dBmV, MER 40.8 dB

  What the modem's own event log shows during each drop:

    "Started Unicast Maintenance Ranging - No Response received - T3 time-out"
    "Received Response to Broadcast Maintenance Request, But no Unicast
     Maintenance opportunities received - T4 time out"
    "UCD invalid or channel unusable"  (repeatedly)
    "SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing"
    "DS profile assignment change" on DS channels 32 and 33

  In-spec levels combined with T3/T4 ranging timeouts and repeated UCD-invalid
  points at intermittent noise on the return path rather than a signal level
  issue. The CMTS also keeps downshifting the OFDM profile, which suggests it
  can't sustain the high-order modulation.

  I have already ruled out everything inside the house:

  - The modem is connected DIRECTLY to the incoming drop. The in-home
    amplifier and all splitters are bypassed. The drops continued unchanged.

 - My router is not at fault - I rebooted it during an incident and the next
    drop happened 5 minutes later.
  - All in-home RF sources (HDMI-over-coax extenders) were powered off during
    several of these drops. They still occurred.

  What I'm asking for:

  1. Please pull the flap list / CMTS logs for my modem - your side has logged
     every one of these re-registrations.
  2. Please check upstream SNR and codeword errors at the CMTS during the times
     listed above.
  3. Please check whether other subscribers on my node are also flapping.
  4. I'd like a maintenance ticket opened for the return path, and a line tech
     with a signal meter to check the drop, ground block, and tap.

Happy to provide account details by DM. Thanks.