Same exact thing that everyone here is describing. I figured out how to replicate the live TV problem.
As everyone knows, our DVRs record the last channel you watch, as well as the current channel you watch. I've found that it happens on live TV only on channels that are recording. As in, if I go back to a channel I was just watching, it goes choppy, even though it's live TV.
So I'll go to CNN and nothing's wrong. I go to HGTV, and nothing's wrong. But if I go back to CNN, it'll be choppy. If I go back to HGTV, it'll be choppy.