I'm having an issue with my mid-2011 base mini. For the last week or so it has been randomly freezing up. The display will just freeze up leaving an odd fragments of the screen visible. All I can do is hold down the power button and shut down then restart. I repaired the startup disk, all was fine. Downloaded and installed the Lion 10.7.5 combo update, ran the Apple Hardware test. Any other ideas where to troubleshoot?