new power supply won’t power up, next?

I have a P-3 computer that was turning it self off and on. I went for the power supply. I got it all booted up. It worked fine for about two hours and then became very very slow. Then it just cut out and would not power up again. I tried another power supply it still won’t power up. The fan on the processor turns one time and cuts out. What do I look for now?? I have not got a clue of where to go from here. I really need some help.