I also have a TivoHD and Comcast's Motorola multi cards. I have been having the exact same problem with missing channels for about 3 months now. For me there are 6 that come and go, channels 808, 819, 821, 832, 891 and 894. One day they are there and the next day I get a searching for signal message. I also have a Comcast cable box sitting next to the TivoHD with a switch for sending either output to the TV. The channels work perfect on that Comcast box. Comcast has been out 4 times replacing cable cards and reprograming them. The last tech said that he has seen the problem before and it was in the Tivo. The only way to prove if he is correct would be for me to send the TivoHD back for repairs, which means they would send me a refurbished one. It just doesn't make any sense how one day they work perfect and the next day they are gone.
S2 and a Tivo HD