Computer Startup Issues

I have problem with a friends computer. It previously had a problem with the motherboard, when the switch was pressed the power supply would start up but nothing would appear on screen. I tested the power supply and it tested ok. Right after it was working perfectly, but when my friend took it home it only worked for a day or two then it developed a new problem. It starts up only after I disconnect the main power cable and connect it back in. It runs for a couple of hours before the screen goes blank and the computer doesn't respond to any inputs. I have to shutdown using the main power switch, but it won't turn back on until I remove the power and plug it back in. TGhe only thing that spins up is the power supply fan.

Im beginning to think the power supply is bad. I hope its not the motherboard again, cause that was just replaced! What do you guys think?