I saw similar results with my Sapphire 2GB (OC). I was seemingly stable at pretty high clock speeds when pounding on it with Furmark, but then Bioshock Infinite and Tomb Raider kept crashing on me. I have since been running at 1135/1350 in order to keep from crashing. I'm not sure what the problem is, but it's not temps. I barely see 70C when running the stress tests.