It sounds like everything else is in order. I would definitely back off your OC on the processor, and any other OC's you might have. See if that has anything to do with it. Also before you update your BIOS you always want to make sure that everything is running at stock speeds before you do.
Try changing the settings you're using in game, turn off AA / AF and see if any of that makes a difference. It could be a bug. I definitely think it is GPU related like Technoholic said, but if the easy stuff doesn't fix it then I would probably go for a BIOS update before you have to start messing with GPU clocks. Normally I would do it the other way around, but because I know they fixed several stability issues with the last few updates it might be related to that if your board has an older bios on it.
Updating your bios is actually quite easy. If you goto the link I provided:
Register an account. Then you can just enter the serial number for your motherboard (It's on the side of the box) or manually look for it. Your board should be a MB-N780-ISH9 but I would double check the box if you still have it.
Then it will give you an option to download support files. The Bios is listed under there and is an CD ISO image. (Version P08) You can burn that with most burning software. Leave the disk in the pc and reboot it. It should automatically do everything for you and then have you reboot once it's done.
If the BIOS update doesn't do anything to help either. I would start looking at graphics cards.