Yes, since it only happens while playing games, it's most likely your video cards overheating.
1. Open your case, remove the video card, use compressed air and blow out the fan and heat-sink with the compressed air. You might even want to clean entire interior of your PC with the compressed air too!. (Remeber not to move or walk arround the room too much while handleing the videao cards as you could damage the card with static discharges.)
2. Use an erraser to clean the gold contacts on the bottom edge of the card, also wipe the same gold edges with a paper towel before placeing the card back in the PC.
3. Replace card in PC and test while leaving the side of your case off.
4. Update your drivers.
5. Install MSI After burner and force your fans to run at 100% (Usually video cards manufacturers will only run the cooling fans at a default speed of 10-40% of max., and at 100% the fans can provide up to 4 times the cooling!)
6. If above works, you maybe able to place your cover back on your PC.
MSI Afterbburner:
http://event.msi.com/vga/afterburner/download.htmP.S. the only other thing that might be a problems is and old or tired powersupply might need replace/upgraded. I've looked at you hardware and your 650 watt PS might be going bad as your close to your power limit!
Try these first, if they don't work repost here or talk with me in Teamspeak 3.
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