Certain games may be stressing your system more then others. Factors that may effect it include game engine type - or if vsync is on or off. I seem to recall farcry2 not having a vsync disable option. Try turning vsync on - on the games that are crashing, (you could even lower the graphics and resolution) and see if the time before it crashes increases, or if the crashes stop all together. This will stress the cards less, helping to narrow down the problem.