Hello I wanted to share something I recently experienced in case it might be useful to someone. About four months ago I built my new computer and was pretty unhappy with it because it would freeze a lot while playing some newer games mostly Far Cry and Silent Hunter3.I did a lot of going through the BIOS then I flashed BIOS with an updated version ( no help) updated drivers, did a lot of google searches and never solved my problem. It made me hate this computer, I wanted to go out and buy another mother board because I thought it was the culprit.(the mother board was the last thing I bought and the new components worked great on my old board). Well, a few days ago while playing Silent Hunter3 (locking up abound) I saw artifacts and it occurred to me that my video card is over clocked Out Of The Box, its a BFG 6800GT OC. I downloaded Coolbits and used it to under clock the processor to stock GT timings and it worked. I like my computer again(Yay!) now I don't like BFG (HA HA) I'm just glad I got it fixed.
Contact BFG about it. You do have a warranty with the card, including it's OC'd speed. I've heard they're pretty good in terms of support.
I think this is one of the first of this nature. I would definately contact BFG and get a replacement, it has to be a faulty card if it can't even handle minimal overclocks set by BFG out of the box.