PSU dying?

I've had a problem a while ago about my hard drive just spinning it self down randomly causing the whole computer to just reboot. Since then I got a brand new hard drive thinking this would kill the problem but it is still there.
No matter what bit of the cable the hard drive is on, it is always Primary Master that will spin itself down.
For some reason I refuse to believe it's the motherboard that is stuffing up, because if it was the motherboard wouldn't it shut off everything that was on the Primary IDE channel?
Could it be a PSU dying? What can it be, I really can't figure it out.

Is it powering down randomly? There's a setting in Windows and maybe your BIOS to stop the HD from spinning after a certain amount of time spent idling, to save power. Is it just doing that when you're idling, or when you're actually doing things?

My computer used to randomly reboot itself. It wasn't random, of course. I think it probably made the hard drive spin down, but I can't recall. In any case, it turned out to be because of overheating. So you might want to check that.

It usually cuts out when I'm "clicking around"(working, surfing etc.), although it has cut out before when the computer is just idling.

I don't think the problem is because of overheating. Sometimes it cuts out right after I've turned on the computer, entered Windows and started some program up.

Try changing the power cable that's attached to that HD? I'm sur there must be a few others spare, I've still got about 2 or 3 left - it could just be that particular power connector is dodgy.

Also check no other components on the motherboard is loose, that can cause erorrs and reboots too.

Okay, tried it out. It doesn't help.
Dang, if it's the motherboard, then I better prepare my wallet for the shock to come.

If it's a power issue, it's unlikely to be the motherboard, more likely the Power supply unit at the top.

It could be possible that you've got too many devices needing power from it, so it can't cope (lke lots of HDs/CDRW/DVDs, lots of fans, and a beefy GFX card). Is it one of those low 200W-250W supplys, or a better 300W or even 400W ones?

Well I just tried a 340 watt PSU and after a few hours it just did it again. It never used to do it. Dangdiddlelido!