Random Restarts! No error messages thrown! HELP!

Hey all, first timer on this site. My PC will randomly restart on its own! Mainly during gameplay like WOW (World Of Warcraft) and BF2. So obviously during stranuous activity. I have replaced my Power supply in hopes that it was jus draining. went from 300w to 430w. worked for a week then this weekend it started again! today (12/5/05) restarted 2 times in less then 7 minutes. When this issue started about month ago, i formatted thinking it would kill it, thought it was a virus. Now a month after the format, it is still at large. Sorry if some of this does not make sense, wrote fast and in dire need of help! Any input would be nice and welcomed!
Thanks
Tanner
P.S. computer seems to run normal since format. Only thing i have noticed wrong besides restarts is on BattleField 2 the game runs great (on low or high video settings) but on BattleField 2 Special Force it runs like crap. choppy, jumpy, the whole shabang. Funny, cause the Spec Force game is an expansion of BF2... no new graphics, just new items and vehicles.
PC Specs...
OS = Windows XP Home. service pack 2
MoBo = Gigabyte AMD athlon XP. dual channel 400+. Nvidia Nforce 2. AGP 8X
CPU = AMD athlon 3200+ 2.19ghz
RAM = DDR 1024g
Vid card = Xtasy ATi 9600xt 256mb
Power Supply = 430w
HD = 120gb
slave HD = 40gb
CD ROM = CD-RW, DVD ROM
and yes. i can somewhat work on these contraptions, but i am definatly not a brainiac. In other words, sorry if this is crappy info on this comp of mine, best i can do

Id say the processor or video card is overheating . Make sure the fans are running right and the heatsinks are clean. The OEM fans seem to fail early. A program like Motherboard Monitor will tell you the system and processor temps.