I have a setup very similar to yours:
G.Skill - 8GB 1866MHz DDR3
Asus Radeon HD5870
I am getting the EXACT same problem, and it is driving me crazy. My motherboard is a replacement for the previous "defective" one that was part of the Sandy Bridge recall. Except the previous motherboard never had this problem. I did a complete re-install, re-seated the ram, made sure to download the newest drivers and still no success. After about 30 min of play I just BSOD. I am playing other games, like Shogun 2, with absolutely no problem. I ran Prime95 to stress test the system and the temps and load were fine.
If you do figure out a solution please let me know.