Since I've installed Lion on my 27" 2011 iMac (12 GB Ram, 2GB ATi video card, 1TB HD) I've experienced a weird problem with the graphics after being on the computer for a while. The shadows around windows turn into weird pixelated lines/boxes, and the Apple logo in the upper left hand corner also becomes distorted. I don't know what's causing this or why it happens, but when I restart the computer it goes away (at least for a few hours). Is anyone else experiencing this? I hope it's temporary and that there is an update to fix it! I included a picture to show what's going on.