Ok, now things have officially gotten weird with his iMac. He purchased a new iMac 2 weeks ago, and moved his old one upstairs, where it was basically going to sit until I got a chance to figure out what needed replacing on it. Care to guess how many times it's shut itself off in that period? That's right! None! Darn thing's working perfectly normal now. He's still bringing it to me to wipe, run AHT and a few other tweaks, but looks like it was the power strip he had it hooked up to that was causing the problem...