I have a iMac 24in, 2.0 Ghz core 2 duo (2007) imac7.1 with 4 GB ram and OSX 10.6.8 (software up to date) and experiencing random freezing. I recently upgraded the ram, but before I upgraded my computer I wiped the HD and reinstalled the OSX and started to have freezing problems. I don't believe the ram is the issue maybe corrput files or? I was trying to do AHT by holding down the D key but nothing happens. I thought it was built into the OSX, apparently it not. Does anyone have any ideas what could be the issue?