Ok, i have been dealing with this problem for like ever and I'm just getting sick of it. Everytime i play a game, it start/loads perfectly, until i get into gunsfights or outside areas which drops my FPS on ( 1024x768 resolution! ). I've tried lower graphics, tweaking the game, tweaking my nvidia CP. But none of it seems to work! I had an 450w PSU so i figure that was the problem. So, then i upgraded into a 500w Antec PSU and Antec 900 case and im still getting this problem!! Its very annoying, when i had my Dell Optiplex 760 it worked perfectly with the games im playing now with lower specs. I tried overclocking everything, still no difference.
Intel E7300 Stock
XFX 9800 GT 512MB
4GB 666Mhz RAM
Windows 7 Ultimate Extreme Edition ( 64-bit )
80GB EVGA HDD ( had a 250GB but it died )