sounds like PSU trouble to me, what size is it (heard of the K7S5A being picky). Also could be due to processor overheating, try and find out what temp ur processor is running at, it could be overheating. I have nearly the same set up as u and under load my processor can go as high as 59 degrees, but still runs stable so not too bothered.
PS. i blow a freebie PSU on that motherboard after only 12 days of use.