I have an early-2008 Mac Pro that's developed more-and-more kernel panics. I pulled my 2nd graphics card (I've been having graphics glitches and thought it might be causing system-wide trouble) and thought it seemed more stable. I upgraded to Mountain Lion a few days ago but today my system is very unstable (crashed while sitting idle, has given me intermittent "flashing power light" problems when I've rebooted, etc.). I have been trying to run the Apple Hardware Test but am not getting anywhere. I reboot with "D" held down and the system seems to just ignore it and "boots through" to the desktop. I've tried to boot from my original install disc (Snow Leopard) and when I hold down the D the screen goes black, shows what may be a cursor, and then ejects the disc and freezes. Is there a new way to activate the Hardware Test? Barring that, is there any way to diagnose a hardware problem from the stacktraces in the system log? I don't want to throw good money after bad on a 4-year-old system and my local service shop charges $80 for a diagnostic, but what are they going to do that I can't?