You have a Wake On Ring/LAN/Keyboard/Mouse function enabled?
No, I built the thing myself and have been all over the BIOS settings looking for clues. I have all the automatic wake up stuff disabled.
Smelled any funny smells coming from your psu (powersupply) lately
The power supply seems healthy. It is a quality unit and runs cool and quiet. My system is plugged into a decent UPS, so it's safe from brownouts.
As a guess i'd say something to do with the power switch, might be faulty, won't be anything software related as if the pc is shut down then no software can run on the machine.
That was my best guess as well. Maybe the start button makes momentary short circuits, long enough to signal the machine to start, but not long enough to turn it off once it is up and running. I guess I will know if this is the cause when I eventually replace my motherboard. If the problem pops up with a new motherboard then it must be the front panel power switch.