Had an identical issue albeit with a different GPU on a Geforce 6800 Ultra (Or similar). That was due to a failed flashing when a previous owner had tried to overclock the card. He sold it as working perfect on eBay to me. Claimed he knew nothing of it but when I checked his bidding history he had bought it in that condition only two months earlier.
In that case I was told re-flashing may fix it though I never found out if it did since I got a full refund.
Obviously this would only be a fix if he has been messing about with flashing etc. Is it possible he has?