Hi RickF,
I believe that occasional pixelation and/or drop-outs in audio when watching HD signals with 5.1 Dolby Digital are simply a function of cable providers shoving so much content down the line (high-speed internet, phone services, hundreds of standard and HDTV channels, on-demand services, multi-language, etc) that it's inevitable that a really wide-bandwidth signal like HD with multichannel sound--even with compression of both audio and video signals--will suffer from time to time.
I suppose it might be an unusually long HDMI connector (if that's what you are using from the Comcast box) or inferior HDMI that might cause it, but I don't use HDMI from my Time-Warner cable box and I still get occasional drop-outs. (On last night's episode of "24", the HD video was fine but I did have very brief audio drop-outs).
Also keep in mind that satellite services are not free of issues. Heavy rain (or snow) can knock out signals and there have been issues of DirectTV using too much compression on some channels in the past, resulting in blocky images and degraded quality. Although it's impractical for most of us, over-the-air terrestrial HD can be spectacular looking and free of video and audio anomalies if you're within reception range.
Regards,
Alan
