The other day my PC just switched off like there was a loss of power or something. A few seconds later it came back on but only for 10 or so seconds and switched itself back off. The motherboard does post but sometimes it can shut itself off almost instantly after pressing the power button and other times it can shut off after a few hours. I don't think the cpu is the problem as it is stable after 30 runs of IBT before this problem occured. I have ran memtest and found no errors in both sticks. The PSU, when shorting the green and black wires on the 24 pin connector does not shut off when a few fans are just plugged in. My guess was that it was either the the motherboard or PSU but since the PSU can power a few fans without shutting off and does not shut off when under load before. After it has shut off I have to flick the swith on the wall socket or the PSU and wait for the power to drain from the PC ( around 3-4 seconds) for the PC to turn back on, I think this points to a motherboard fault.