Download and test the memory with memtest from
http://www.memtest86.com/ . If the machine is only rebooting, chances are it's not the motherboard or processor. It's a power problem (which you switched out the power supply) or a memory problem. If your memory checks out ok, what I'd do next is bump up your voltages on your cpu and memory just a little and see how that works out, but if you haven't much experience with that, you can damage hardware doing that, so do so at your own risk. Good luck.
Scott