Motherboard/CPU overheating problem

I recently got a computer from my friend because he couldn't figure out how to install windows on it. The first day I had it I was able to play games and surf the internet with no problems at all. And then it started shutting off randomly. My first thought was to reapply the thermal. 99% of the time when I've had problems, that's solved it. No luck this time.

So I downloaded core temp. And at idle it says my cores are roughly 36c and 40c. As soon as I started a game, the temps jump up to 60c+ and keep rising. Which threw me off as it rises about 20 degrees the moment the game opens, which seemed a little too fast to me.
So I thought perhaps my heatsink cooling fan was not functioning properly, and replaced that. No go. I bought a new heatsink entirely instead of the stock one. The Arctic Freezer 7 pro rev.2. Still shuts off.
I changed the power supply for one I know works. Still shutting off. I noticed that my motherboard diodes are getting extremely hot, and positioned a fan onto it and now my temps do stay a bit cooler, but still not a safe range.

Specs
AMD Athlon X2 dual core 5600+ 2.91GHz
gigabyte S-series motherboard (unsure of exact model at this time)
2gig RAM
NVIDIA GeForce 9400 GT
Windows XP pro service pack 3
4 fans, plus CPU and adjustable fan on my power supply.


I've gone into the bios and everything is getting the correct volts.