I know it's been mentioned here before so try a search for that problem on these forums.
I've seen it a few times myself and usually simply changing channels away from and HD one, and back, seems to fix it. It might be something about the buffer filling up.
Also, I recall mention of going into the menu and switching the output to 480p for a moment then back to 720 (or 1080) to solve the problem if it persists. You should find that info via the search.