Home Network Troubleshooting: Xfinity, TP-Link Mesh & Cable Modem Diagnostics

I’m having intermittent Internet slowdowns and occasional loss of connectivity. I’ve already replaced the indoor splitter, the coax from the splitter to the modem, and the Ethernet cable from the modem to the router.

My NETGEAR CM1000v2 modem logs are showing recurring DOCSIS errors, including:

• T3 ranging timeouts
• synchronization failures / loss of sync
• failed QAM/QPSK symbol timing
• DHCP registration failures
• Dynamic Range Window / upstream power-related warnings

On at least one occasion, the modem lost registration long enough that my router stopped receiving its normal public Xfinity IP and instead got a temporary 192.168.100.x address from the modem.

I’d like you to check the RF side of the connection, especially the upstream return path. Please check the drop, outside connectors, tap, signal levels, and whether there is any ingress/noise or intermittent impairment on the line. I’m not looking only for a modem reboot or reprovisioning unless you find that is actually the cause.

If the line tests clean, I’d also like your opinion on whether the modem itself could be contributing, given its age and the repeated re-ranging/synchronization events.

The issue is intermittent, so please look at historical signal/error data if available, not just whether the modem happens to be online at this moment.

I understand the levels may be within spec right now. My concern is intermittent upstream/ranging instability. Can you check for historical T3/T4 events, upstream noise, flap history, or problems at the tap or node?