Weird Power up problem...

[TOCSYS]

I try to turn my PC on and nothing happens. So I have to open my case up, and with a power supply tester in hand, I have to "jump start" my PC. (Take out the 20pin connector and plug it into the PS tester.)When I do that the system turns on and the fans start spinning.(I have an I850 motherboard so it has 2 other power connectors going to the board. I guess that's why the board gets power when I hook the main 20 pin connector up to the tester.) After that, I plug the 20 pin connector back into the motherboard, close up the case, hit the power button and everything starts up fine. I'm good for the whole day until I go to bed and come back from school which is about a 13 hour period. When the system is running I can(but I don't)overclock my 1.8Ghz P4 to 2.1 with a volcano cooler and default voltage. Can I assume that the problem is the power supply or is it somthing on the motherboard? Maybe even a setting in the BIOS? Check my main rig out for what I'm running. BTW, it's a 350w Ennermax.