This is a huge issue for me. I can't play videos reliably anymore. It doesn't matter if it's in QuickTime X or iTunes, but after I play 2 or 3 videos, the system starts to freeze up and I get a spinning beach ball cursor. After this happens, all videos are broken until I restart. There's a pretty lengthy discussion going on Apple's discussion boards called 'OSX Lion + 2011 iMac 27" + Video = Freeze' This sucks.