Okay, could one of several things, but start by making sure all your fans are working, case is staying cool, GPU, CPU, fans etc. Make sure no dust build up on fans or heatsinks, inside the video card. Try pulling 1 stick of memory at a time, try launching the games and running them with only 1 stick of memory, then try the other stick by itself, to rule out memory errors.
What is your PSU situation? Sometimes a failing PSU will rear its ugly head when demand is placed, like launching a game.