power supply problem

[elevationzero]

I recently upgraded from a 250w power supply to a 400 watt supply and ran into a strange problem.
At first glance everything worked fine. But when i shut the computer down from windows and then go to restart the comp. The following happens, All the fans come on, and the drives appear to power up but that is it, the computer wont boot, If I turn off the switch on the power supply or pull the plug for a moment and then plug it back in the comp will go ahead and boot up just fine. I can restart the comp just fine, I only run into this problem when i power down the system.

I took the 400w power supply that was giving me the problem back and exchanged it plus a little more money for a 500w supply by a different manufacturer, Exact same problem again, If I go back to the 250w supply everything is normal again. The only difference i see between the old power supply and the newer ones is that the older one only has atx connectors while the newer ones have both atx and p4 connectors. I dont see any switches on the power supply to say choose atx as opposed to p4 etc

System specs are as follows
asus a7v2666-e mobo
amd xp1900 proccessor
2 ibm desktar drives in raid
512 mb ddr memory
win xp
geforce 2mx

thanks for any help

 

[Shooters]

So why don't you just go back to the 250 watt? Were the 400watt and 500watt the same brand? It sounds like a flaky PS if you ask me.