That's pretty strange - have you tried forcing a system update on the cable box? Comcast should be able to tell you how to do that.

I have the exact same issue... The trouble started about 4 months ago. While it doesn't happen all of the time, it does happen a lot more often than not.

I have contacted both Comcast and Tivo to see if a new software patch was released - and both say no. I've even gone so far as to have Comcast replace one of their boxes. (by the way, I have two of these both doing the same thing, and both have been working flawlessly for oh, I don't know, lets say 8 to 10-years.... until about 4 or 5 months ago.)

I would love some help with this issue myself.

BTW, My configuration is:

Tivo Series II
Motorola Digital Cable box (non-HD) via Comcast
Serial connection between them for channel changing
Tivo Wireless network connection

You might try going through guided setup again and use IR blaster cables. If TiVo/Comcast fixes the issue, you can always go back to serial.