I have been having this problem for about two weeks now. The cable shuts out and there is an error message that reads "One Moment Please, This channel should be available shortly. Ref code: S000". Except the channel does not return, and the picture is often bad quality and choppy with both audio and video. I power cycle the cable box and then the picture and audio comes through as if normal, but the guide takes a while to load. Then the problem happens all over again after less than an hour.
The "One Moment Please" message is misleading. It is generated by the cable box, which has no idea if or when it will be able to display the channel again. All it knows is the signal for the channel is not usable. Please see http://forums.comcast.com/t5/XfinityTV-and-Equipment/ref-code-S0a00/m-p/1155673#M42454.