Intermittent outages — S34 upstream at 54.5 dBmV, repeated T3 / Loss of Sync / TCS failures

I'm experiencing intermittent internet outages and have isolated a DOCSIS upstream/return-path problem at my ARRIS S34.

Current signal levels:

• Four bonded SC-QAM upstream channels: 54.5 dBmV each
• OFDMA upstream: 48.5 dBmV
• Downstream power approximately -3.5 to +0.1 dBmV
• Downstream SNR approximately 40.4–45 dB

The modem event log is repeatedly recording:

• No Ranging Response received — T3 time-out
• SYNC Timing Synchronization failure — Loss of Sync
• TCS Fail on all Upstream Channels
• RNG-RSP CCAP Commanded Power Exceeds Value Corresponding to the Top of the DRW

The modem and router are both on UPS power, and the modem-to-router Ethernet link is stable at 2.5 Gbps.

There is approximately a 10-foot coax extension inside the residence that I have not yet had time to bypass-test, so I cannot completely exclude an in-home coax contribution. However, the DOCSIS logs clearly show repeated upstream ranging/synchronization failures.

I would like the upstream return path, drop, connectors, and levels from the modem back to the tap/plant checked, rather than only a modem reboot or provisioning reset.

The problem is intermittent, so the modem may appear online when checked remotely.