I'm also curious how this turns out. I had my card in slot 2 of my Series3 fail a couple months back in the same way. Couldn't see any HD channels other than local programming. I did multiple tech-support calls with attempts to 'hit' the card and restart the Tivo. None of them worked permanently. After an hour/day it would be back to missing channels. I ended up getting a new card installed and that 'failed' after 24 hours. Now, I can consistently eject the card in #2 slot, reinsert it, and restart Tivo to get my channels back... for about 24 hours.
I'm very inclined to think that this is a problem with Charter (I realize the OP is on Comcast) rather than tivo, but I did find a a thread here
that suggest that the slot might be bad?!?!?!
In my case, I get a gray screen and as far as Tivo is concerned, it's receiving the channel. When I truly don't get a channel, it kindly tells me so onscreen.
In any case, please keep us posted as to your resolution.