When you OC, heat is your common enemy. Although you will pass all the tests but if things gets too hot, you'll eventually lose stability. Like what gengstapo mentioned, the problem might most likely be a hot graphics card because if you pass Orthos smallFFT/Blend for 9 hours, your CPU/RAM should be good. Also when it crashes, does it give you a blue screen or it jumps out of the game back to Windows desktop?