Thanks -- that helped a lot. I had turned it on based on someone's post in this forum, but turning it off got rid of all of the artifacts I was getting on NFL games.
The only problem I'm having now is that WGN's HD feed of today's Cubs game is just horrible. Most of their HD broadcasts are fine, but today it looks like I'm watching the game through a dirty window. Every other HD channel looks great, so it's an issue with the feed. And by the way, last night's MPEG-4 test on D* looked absolutely stunning -- can't wait for the full channel release.