Being a loyal Boilermaker, and by definition, a ND hater, I gleefully watched every second of the game on a Hopper. I saw none of the artifacts that the OP saw, or maybe, since I was enjoying watching the beat down so much, my mind blocked out the bad stuff.
I've experienced the same as the OP a number of times, but I've found when that happens--for some unknown reason this impairment happens--I have to tune to another channel, and then tune it back to the original channel with the impairments and all is fine. It's annoying to have to do it, but for some reason IT WORKS. Swapping won't work; one must change channels on the tuner, and then change the channel back to the original channel and it fixes itself.