iMac 20 inch early 2008 6 GB ram 500 GB free space The last 3 days the screen has frozen on me for no apparent reason. I reinstalled Mtn Lion and it still did it the next day. I have run DiskWarrior and Drive Genius - all is well. It happens when I step away for a few hours and return to have the computer completely locked up. Have to power off to restart. Any suggestions?