HIS 6970 graphic card 8pin power prevents pc from starting?

my friends 6970 gfx card doesn't allow me to start my pc up at all. but when i have it connected to the 6 pin pc does start. of course i need both connectors to make the graphic card start so i wont be able to see any thing with 6 pin only. does any one know why it doesn't start with the 8 pin? power supply is fine because i have a 6950 his version works just fine. i have a 1000w kingwin mach 1 psu and a msi 990fx-gd80 mobo.