Every 11 days lost Internet connection

I have the same problem, although it's usually on Sunday evening (Monday morning, 12:15am). I suspect although have no hard evidence that it's Comcast updating the modem's s/w. For whatever reason, they don't or cannot complete the provisioning process and so we have to power cycle the modem. I'm actually in the process of writing code to scrape the modem's event page logs and look for the sequence of error reports to know that it should be rebooted. My family will tell me "the wifi is down" when in fact, the wifi is fine and the modem needs a power cycle.

I too have the exact same problem with a T25 modem I bought 3 weeks ago. It dies overnight with a "SIP Registration Timeout" in the Event Log. It has happened three times, 5/6, 5/12 and 5/22.  Comcast Tier 3 support suggested that I return it. Spoke to Arris today; still more diagnostics to do.  Does anyone know what the latest firmware should be? Comcast reset the boot parameters on my modem last week, so I'd hope it is up to date, but Arris says only Comcast can tell me the right firmware.  (Huh?)  Also, I have "Extreme 600" (MB) internet speed, but haven't yet seen more than 250 Mbps (no better than the Arris TM722G it replaced).

My firmware info:

Firmware Name: TS11.01.142_061719_701.PC20.03
Firmware Build Time: Mon Jun 17 15:46:15 EDT 2019

That's the firmware I have. I'm really suspicious of all of this. I tried to pursue getting into the advanced side of the modem with both Arris and Xfinity and got a total run around. Each pointed at the other claiming the other was responsible for locking me out of the modem. Clearly, Xfinity and Arris have an agreement via the DocSIS API that Xfinity can load s/w onto the modem that I cannot control even though it's my modem bought with my money.

Okey Dokey - that's our agreement. However, if that's so, you (Xfinity) own all of the problems and can't blame the vendor or me or put work on me to fix things out of my (our) control.

Basically, it's extortion for not paying the montly contract rental fee. Mildly flawed service and not support when it fails.