If this only happens during intense gamplay, then it is probably one of three things (or a combonation of them). First, how big of a power supply do you have, your system might be drawing more power than your PSU can put out. Second, whats the temperature of your case, more specifically, your GPU (video card)? You might want to look into buying a GPU cooler if thats the problem. Third, I noticed that your CPU is a 1.9 Athlon, no offense but this is definantly not up to par for high end gaming. If you can spare the cash you will want to upgrade to a faster processor, as this may also be contributing to the problem.
If you could, I would like to know the temperature of your video card during gameplay, as well as the wattage of your PSU. With this info I can help eliminate some of the causes I listed above.