iMac Shutdown Issue?

So I got an iMac late August, and for the past few months it's been randomly shutting down. Sometimes it'll do it while I'm using it, other times I'll come back to it after a period of time and find that lovely error message I'm coming to recognize far too well.

I Googled this and the only thing I found was someone who said they had this issue and had to get a new Mac. I took it into the Genius Bar a few weeks ago, and they had it for a few days...diagnosed it as a power supply problem. Within a day of having it back, it did it again. And again. Sometimes it's daily, sometimes a few times a week, sometimes not for a few weeks. It's very odd.

I'm also not super thrilled at the service I've been having at the Apple store and through Apple Care, so before I drive another hour one way to have them look at it again, I'd like to see if any of you have heard of this issue.

Generally random shutdowns are either corrupted SMC (resetting it's as easy as powering off and pulling the power from the back of the iMac, leaving it out for a minute and plugging it back in). Or hardware issues. In either case, try the first one, if that doesn't resolve it reopen your case with Apple. Call them on the phone, build a case record, and go from there. Also, do you have a UPS, any possibility of momentary brownouts?

I don't, but some of my programs will freeze...