Does any of the other system sound like it is losing power? Maybe at boot up you hear it start then it sounds like it isnt gonna boot then it regains power and boots up. If that then maybe you are drawing too much for the PSU. But 400 should be fine, unless you haev added several extra things like a scanner that draws power.

Instead of the PSU change, try uninstalling everything from the system that has to do with the card. Remove the card from the MOBO. Then start from scratch and reinstall everything.

I have had this happen to a friend of mine, although his was just it randomly blacking out. I think sometime ATI has conflics with different things that you can install. Is there anything new that you have added? DO some of those things and check to see if it will work again.

Tyler