Given that the problems occur when gaming the first thing I would look at is a possible heating issue. Keep an eye on the temperatures (using software such as Motherboard Monitor), make sure your fans are working properly, clean out your system with come compressed air making sure the heatsinks are clean, try leaving the case open, and even consider using a small box fan to blow air into the case while gaming to see if it makes a difference.

Aside from that I would:
1.) Make sure you have the latest drivers for your graphics card.
2.) Try swapping out the card with a friend's and see if it makes a difference.
3.) Make sure that your PSU can handle the load your graphics card and other components put on it while gaming.

Let us know how it goes.