Computer turning on and off after swapping psu

Here is the problem. I have an HP Pavillion and I was trying to upgrade the video card. The upgrade required a new power supply.

Replaced:
gt 620 with gt 560ti
400 watt with 550 watt

After all was said and done the computer would not power on. Checked the power supply using http://www.techspot.com/community/topics/tutorial-suspe... method, it works great.

Swapped back to old video card, computer turns off and on intermittently.

Swapped back to original set-up, computer still turns off and on intermittently.

Thought it might be a ram problem. Tried removing each dimm separately had the same issue of turning off an on.

Also unplugged just the 24 pin connector from the mother board, connected ground to ps_on (green) and the hard drive and cpu fan started up fine.

Replaced the thermal paste on the cpu and heat sink... still same problem  

Anyone know what the problem might be? I'm guessing (but hoping I'm wrong) that something on the motherboard got short circuited.