PC powers off after powering on

Hi,

I have a Gateway PC I am working on and the problem is when you press the power button you hear the PC start up. Then after about 3 seconds it powers completely off. I have replaced the power supply with a new one, removed the cards one by one including memory and testing after each one was removed. Still the same thing. I have also used different powers cords and outlets as well. Anyone help will be greatly appreciated!!!!! Thanks,

Dana