my iMac has been pretty trouble free for most of its life. i had the dodgy 6970m graphics card, but apple replaced that plus also the screen itself as it failed at the same time, that was maybe 18 months ago. now just in the last week ive noticed it randomly restarts. its sometimes when im using it, but also since i always leave it on, i know it does it when im not in as ive left apps open and they've been closed when i came in. it runs for hours of normal usage, and then suddenly ping, screen off, instant restart. only once have i really noticed it do it frequently. about 3 nights ago it did it 3 times in 30 minutes, but normally i sit all evening without it doing so but then the next day it will do it once. i worry its the power supply but theres not much plugged into it, one external usb drive, and i use it for charging my iPad. however ive just recently ordered a thunderbolt ssd which should arrive any day, guess a clean install on that will tell me if its software or hardware. any ideas?