Recently I was hit by the Tivo Grey Screen problem which seems to be brought on when using analog cable using a new Tivo HD unit.  The best explanation I've seen on this Tivo analog grey screen problem was found in the Tivo support forums.

The 11.0 software introduced a serious bug that affects those with basic / analog cable. This bug causes the analog tuners to "go out" periodically, resulting in a black (or gray) screen on analog channels. To regain the picture on analog channels, the user must switch both tuners to digital channels (or switch between a few channels on the Settings -> Channels -> Channel strength screen)...or alternatively, reboot the TiVo.

Note this issue does not affect the digital tuners, so it does not affect digital channels from an off-air antenna or cable. When you install a CableCard on most cable systems, you get digital versions the analog channels, thus eliminating potential issues with lost analog tuners.

According to customer service, TiVo is working on an update to eliminate this issue for customers with basic / analog cable, but it's unclear when that will become available. One customer was told that TiVo expects to resolve this issue for good by the end of April, but I have not seen any other confirmation of that.

I just ended up calling my local cable provider and switched over to digital cable with a cable card.