Garbled Channels

Can anyone shed any light on why I would have a few garbled HD channels on an intermittent basis? Specifically, it is four channels in close proximity to each other (432, 434, 438 and 439). I've swapped cable cards without success. Comcast is blaming the Tivo (of course) but I'm not convinced since it is an intermittent problem. Here is an example of the diagnostics screen for channel 432 at a time when the signal is garbled:

Tuner: 0
Channel 432
Frequency: 79000KHz
Modulation: QAM 256...

Garbled Channels

Garbled Channels