Whenever I shut down my computer, or put it in sleep/hybernate, it turns on after two seconds. It doesn't post, it just powers on and then idles. To actually turn it off, I switch off the psu. The problem is now, whenever I switch the psu on and try to boot, it doesn't always turn on. It takes a good amount of flicking the psu switch on and off before the motherboard lights up.

So far I've determined the things its not:

• its not caused by the mouse or network waking up the computer. I've been able to go into hybernate for the past year. And all "wake on X" settings in the bios are diabled.
• its not a scheduled task waking up the computer at a given hour, it occurs every single time
• its not due to an upgrade or new installation, since I haven't done either in a very long time

I'm sure its a hardware issue. So I'd like to know, is my psu dead, or the motherboard? The psu is an Antec Earthwatts 600w, the motherboard is an Asus P5Q-E, both one year old.