Shutdowns

I have the first generation aluminum iMac, I believe it's coming up on three years old. About a year ago I began having problems with my Mac shutting down while I was in the middle of using it.

I brought it in to the Apple Store and the manager removed a preference folder from somewhere within my system folder. For the longest time I had no more shutdown problems, until just recently.

I was wondering if anyone here has any ideas what may be causing this, and what preference folder (and location) can be removed to possibly prevent the shutdowns. I'm assuming something might be getting corrupted in that folder causing the problem. I often run the disk check and it finds issues and corrects them, but I don't think that helps with preventing the shutdowns.

Any suggestions will be appreciated. Thanks!!