Xfx 6850 black screen

my friends computers been acting up recently her computer will randomly black screen on her at first i thought it might be the power supply but the computer doesnt shut off the screen just goes black and loses signal the card calls for 500w min that is what she has the card also said it only needed 1 6pin to run but it wouldnt run with just the one it needed both if some one could help out that would be great