I live in center city philadelphia and have had many problems with comcast, as well as comcast service. I had HDTV cable hooked up last fall, and went several months with constant video and audio breakups (often in combinations of 2 channels, similar to how you describe). It seems it has something to do with how they broadcast, or something. The 4 channels are each connected in 2's somehow, so the signal usually is not sufficient on both channels if there is a problem.
I had service techs come out on about 8 occasions, each time they really knew little about HDTV, and had NO idea how to fix anything. I called several supervisors, and got no replies (for months). It was only after my posts on here got seen by a Comcast Marketing executive from another state that I got some action. He was nice enough to give me his number, and within 15 minutes, I got 2 calls from the local supervisors.
They came out to my location, and could still not fix it. It was only after they fixed something on their end (from their central location that it resolved, and finally works acceptably (occasional breakups, but at least not every few seconds).
In summary............don't expect very good service from comcast, unless you really push them, and the problem could be on your end (possibly connections, or the signal strength not quite strong enough to your location), or, as in my case, it may be a problem on their end.
Also, when they come out, insist they bring their meters out to test the strengh of the signals.