For the last month, my computer would shut down at random times. When it shuts off, the main power button doesn't work. The only way I could turn it back on is if I turn the power supply switch to off for about a minute then turn it back on. After that, the main power button will work. Sometimes it works for three days, others for five seconds. I'm using speedfan to monitor the temps, and nothing is overheating. Is it the power supply or the motherboard? Any ideas?