I'm in this boat as well, but it's not even with mission control being open. It seems to happen on Chrome when I have a couple tabs open that have a lot going on, but Chrome will freeze for a second or two and then the top of the window gets all fritzed out. Looks like colored TV static. A quick hide and re-open fixes it, but I can't tell if it's something I can actually fix, or just a symptom of the problematic 2011 MBP graphics cards.