OK so temps aren't an issue, good. At least the one you have has active PFC and is 80+. I would try another PSU to see if it fixes it. I'm not sure how long you've had it, but yours may be starting to wear out. Get it somewhere locally so you can return it, fry's for example. if that's not your problem. Other than that. have you made any changes to bios or disconnected/reconnected anything? just a thought.