I am thinking that it can be 1 of two things:
1) The connector on the card is messed up
2) The connector from your PSU is messed up
Do you have another psu that you could test it with?? Or maybe another rig where you can move the card to and check it there???
I dont doubt ur psu can handle it cause i had a 297 watt (£15) psu and that ran the card (+2 HDD, 2 DVDRW, 2GB ram, 3ghz p4) without fault for over a year ( PLEASE NOTE I WONT RECOMMEND USING A SHIT PSU TO ANYONE :P )