Intermittent connection drops during calls; modem log shows T3 timeouts, sync loss, and downstream uncorrectables

Hi Xfinity,

I’m having an intermittent connectivity/stability issue, not a simple speed issue. The connection briefly drops in and out, which is enough to disrupt voice/video calls. When the service is up, routine checks can still look normal, so this seems more like a reliability problem than a bandwidth problem. Xfinity’s own troubleshooting flow also distinguishes between general connection checks and cases that may need deeper troubleshooting or a technician if warranted.

I pulled the diagnostics from my modem, and a few things stand out:

• Repeated critical log entries for “SYNC Timing Synchronization failure - Loss of Sync"

• Repeated critical log entries for “No Ranging Response received - T3 time-out”

• A noticeable number of downstream uncorrectable codewords on multiple channels, including 483, 531, 573, 591, 609, 615, 651, 669 MHz

• Downstream power is roughly +2.0 to +6.3 dBmV with SNR around 37.3 to 37.6 dB, and the OFDM downstream channel shows +7.0 dBmV and 38.4 dB MER.

• Upstream bonded channels are around 48.5 / 49.8 / 50.5 / 50.5 dBmV.

For reference, NETGEAR says good downstream power is -7 to +7 dBmV and good upstream power is 38 to 48 dBmV. My downstream snapshot seems to look okay, but upstream transmit power appears to be at or slightly above that guidance on several channels. 

I also found Xfinity forum discussions where recurring T3/T4 timeout patterns with otherwise decent downstream readings were treated as possible upstream noise, return-path, drop, or plant-side impairments.

Could someone kindly review this for:

• line stability / disconnect history over time

• any upstream or return-path impairment

• drop, connector, splitter, grounding block, or tap issues

• any area impairment that could match intermittent brief outages

I’m hoping for a review beyond a single “signals look okay right now” snapshot, since the issue is intermittent and the modem log does show repeated T3 timeouts, sync-loss events, and downstream uncorrectables.

Thank you.