Yes, it could be shorting on the bottom of MB or you might not have a good bond on the HSF cooler, too little or too much thermel paste will act like that. If it comes down to it, remove the cooler, clean both surfaces with 90% alcahol, and then use 1 bb sized drop on one of surfaces and mate them again.
Had this same problem a few days ago when I put my new pc together. Totally pissed me off.
The new power supply I got (680wHiper newegg.com) had those fancy blue sleeved cables and bundled together was the ATX motherboard power cable and the 12V 4pin connector. I plugged those in and it would turn on for about half a second. Not long enough to show anything on the screen and then it wouldn't turn on again unless I cut the power to my PS.
Anyways, after staring at it angrily for a while. I looked in the spare wires I had left over(they disconnect)I noticed another 12V 4pin connector. I unplugged the one that was sleeved together with the ATX connector and plugged the spare into the "P4" slot on the PS and hooked it up to the motherboard. I didn't think this would do anything but it booted right up after that. Worked great ever since.
Probably not what's going on with you if you've used more than one power supply. Also, check your motherboard manual and see if it has an overheating CPU safeguard feature. I once bought a shuttle board off newegg and when I got it, the same thing was happening. I looked in my case and saw a red LED by the CPU and looked it up in the manual and it was the light that comes on when your cpu is overheating and it shuts down the pc and doesn't allow it to boot up. It was malfunctioning so I sent it back and they gave me another one that worked just fine.