I've looked for similar issues here in the forums but haven't been able to find any.
Every 3-5 weeks my XL4 loses cable card pairing with Comcast. This has been happening since I bought it last August. I usually lose all non-local channels, but the last time I actually lost all HD locals as well.
At first I thought it was the Cable Card -- I had 5 of them replaced. Comcast checked our cable signal and it was perfect. I also have a Series 3 and a PC with a Ceton card and those cable cards never get unpaired -- only the XL4.
I can fix the problem most of the time by pulling the cable card out, reinserting it, and having Comcast re-pair it from their head-end. But after a few weeks, the problem occurs again.
I'm thinking that it's a bad TiVo since my Series 3 and Ceton work just fine with their cable cards. I even have the XL4 on a UPS.