Have you got the 2x256 in the first two slots? And the 1x128 in the third slot. Also check to make sure your CPU, (Processor) is not getting too hot, same applies to your graphics card.
I don't play any of the games, but someone has mentioned that the "Sims game" is quite demanding on CPU, and graphics card.
Remove side of case and use a fan to blow on the graphics card while playing the game and see if it still happens. J.