Unexplained, random power offs.

The SMC! Thank you. Hadn't thought of that. I had to re-set it once a year or two ago when the fans came on at top speed.

Several hours and no random restart. I'll report back.

Within moments, literally, the iMac decided to spite my words and shut down.

Sounds like a bad power supply then, have you tried my other suggestions?

I put a reconditioned power supply in yesterday. No luck in solving the problem. Everything was fine until a few moments ago.

I'm beginning to think our house may have an electrical problem, which is odd. i've only been here a few months but haven't had any issues up until this started a week or two ago. No major changes either. I just ran an extension cord from a different circuit to the iMac in hopes that I can just get it to work long enough to finish my thesis without remembering to save after every three words. I'm also going to buy a voltage regulator to see if that does any good. A friend just suggested there may be surges that are tripping the power supply to shut down the iMac but are too low to shut down the power stip it was on.

Solved. I purchased a voltage regulator and the iMac has now been on for nearly four days straight with only one reboot that I initiated. In related news, I learned the wiring in my house is terrible and will likely need to be replaced. Thanks for the help.