This seems to happen when I watch a HD YouTube video on my 27'' 2011 iMac. What happens is the bottom half of the video gets yellow lines and flickers a bit. A refresh fixes it but once it happens my whole OS X graphics get screwed up. Here is a picture to show what I mean. As far as I know only a restart fixes it. Doesn't seem to be occurring in Windows, only OS X Lion. Any tips?