So my computer started off shutting off randomly a couple months ago. I would be in the middle of something and then it would just shut off, not shut down, and then it would sometimes restart itself sometimes not. It would happen sometimes twice a week sometimes once a month. Last night however it happened nine times. I booted into BIOS and it happened there so correct me if I'm wrong but that should rule out the HD's? I'm thinking its either the motherboard or the PSU. Is there any way to tell other than trying new ones of each? I don't have a spare PSU so I'd have to buy either one and don't wanna get the wrong piece.
It more than likely the power supply. If not then possibly the motherboard. What are your system specs and what type of power supply are you running? How long have you had it?
What country do you live in? Staples has a wonderful "No Questions Asked" return policy. You could try a power supply from them and if it still doesn't work you can take it back for a refund within a week.
Sounds like a plan thanks. Though there are times when it doesn't happen for days, possibly even a week or so. Is there any other way to rule out the PSU other than trying a new one? I guess it even if it happens with the new one it doesn't hurt to have an extra psu lying around for such occasions.
Go into BIOS and under PC Health or PC Monitor something along those lines it will list all the temps and voltages. Give the voltage it says for +12v, +5v, and +3.3v. I'll tell you if the voltages are within tolerance.
Those look to be within a decent enough range. It could be that they dip below tolerance at the times when you get the random shutdowns. I still think it is a good idea to try a different power supply.