I would suspect perhaps the card is either overheating a little bit, or you have the settings up too high...
1. Have you dusted your PC case lately? If you haven't opened the case since you got the card, I would open it, take out the card, and get all of the dust out of the fan and heatsink... This would be my number 1 suspician... dust likely causing overheating. Perhaps the fan is filled with dust, and stopped spinning...
2. What are the specs on the rest of your system? (Mobo, CPU, RAM, OS, etc. etc. etc.)
3. What settings level do you have the games set for? If you have already dusted the inside of the case, then set all of the game settings on lowest possible... from there, work them up until just before you have the issue.