Power cycles on and off [Custom Build]

Hello,

I have a 3 year old system which has always had intermittent issues starting/rebooting.

It will sometimes fail to start (with no beeps). The fans, and hard drives will spin up for 2 or 3 seconds, the machine will power off, then power back on. This cycles continues until I unplug the power.

Once stuck in this cycle the only way to get the machine to start again is to physically unplug the power supply and wait for a few seconds.

After the machine is running there are no issues.

Does this sound more likely to be a problem with the power supply or the motherboard? I've ruled out most other components.

Machine Specs:

Power Supply: 750 watt Corsair
MB: Asus p5ke-wifi ap
4GB Gskill
Nvidia 8800 GTS 640mb

Thanks