Remove your card and reseat it.
If that doesn't help, check the power connector.
It could be that the slot is bad.
What mobo is it? Have you installed the chipset drivers? Possibly could need new BIOS, but I doubt it as the 6800's are older cards.
My Rig: SABLE
Antec 300 Illusion / Antec EarthWatts EA650 650W / EVGA GTX 760 2GB
AMD FX 8320 x8 Black Edition / Gelid Tranquillo / MSI 970A-G43
Sandisk Ultra Plus 128GB / Samsung 840 120GB / WD Black 750GB / WD Green 1TB / WD Red 3TB
G.SKILL Ripjaws 2x2GB DDR3 1600 - Crucial Ballistix Tracer 2x2GB DDR3 1600
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R.I.P. Danny L. Trotter
... 14 Nov 1945 - 4 Sept 2009