I know this is an old post, but for those researching this same issue, this may help.
I have had this same problem, and after checking cooling, cpu, power supply, memory, and video card, it turned out to be a short in the reset switch (either in the switch itself or in the motherboard). I simply unplugged all of the switch wires from my MOBO (reset, HDD LED, power LED) except for the power switch wire. Problem solved! you may try this before purchasing any new equipment.