Intermittent DOCSIS loss of sync / T3 timeouts causing severe packet loss

I’m experiencing intermittent Internet failures where connectivity slows to a crawl or effectively drops for approximately 5–10 minutes, then returns to normal for an hour or several hours before happening again.

This issue occurred with my previous modem and continues after replacing it with a new Hitron CODA56, so I’m concerned there may be an RF/drop/return-path or plant issue rather than a router problem.

During the failures, the CODA56 logs repeated critical DOCSIS events including:

• No Ranging Response received — T3 time-out
• SYNC Timing Synchronization failure — Loss of Sync
• Lost MDD Timeout
• RNG-RSP / CCAP commanded-power warnings

We have also observed the modem’s upstream transmit power reaching approximately 55–57 dBmV during problematic periods. After tightening a slightly loose coax wall connection, upstream levels improved significantly into approximately the 38–44 dBmV range, but the intermittent T3 and loss-of-sync events have continued.

The downstream signal generally looks reasonable between failures, approximately +4 to +8 dBmV with roughly 39–41 dB SNR, but the modem has accumulated substantial corrected and uncorrectable codewords.

I have independently tested my internal network during these incidents. Local traffic between my router and servers remains sub-millisecond with no packet loss, and the modem’s Ethernet link remains up/full-duplex. During the DOCSIS failures, however, Internet latency and packet loss become severe. After the modem recovers, Internet latency returns to normal.

Because this is occurring across two different cable modems and the modem itself is recording ranging/synchronization failures, I would appreciate having the Comcast side checked for:

• CMTS/modem flap and ranging history
• Upstream return-path health / ingress or noise
• Signal levels at the tap and drop
• Outside coax/drop condition
• Node or plant issues affecting this connection

If the remote diagnostics do not identify the cause, I would like a line technician to check the signal at the tap/drop and escalate to plant maintenance if necessary.

I’m happy to provide modem event logs and current signal/channel readings if needed.