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
Ok, I have a Hercules Radeon 9800 XT. Something is going wrong with it. The colors on my screen are becoming distorted (random lines of color are appearing). I also hear this sort of click noise all the time, that i used to only hear on boot up. Sometime the entire screen goes black for an instant, and then reloads, but with random black spots. I was wondering if this a hardware or software problem (i have all the latest drivers). The only other thing i can think of is, it might be the PSU. I have an old (about 4 years) 400W PSU to run the card, my Athlon 64 3400+, 2 hard drives, 2 cd drives, and the assorted fans and LEDs i have. Could this be causing the problem?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic