Does the mobo have onboard video too? Try using that and make sure it's not trying this card first.
Check the PC speaker connection. Maybe it's trying to beep and we could work around some error codes.
Does it cut off by tapping the power button, or must you hold it down?
Um...as usual with this kind of problem, be sure everything is seated proporly, perhaps remove some stuff if you can to see if anything is cauing some conflict...