Games crash to desktop

Game crashs to desktop
I dont know what to do, all my games keep crashing. KINGPIN, MOHAA, CS, Quake2. The games start fine and i get a few mins of play but then when the game play gets going fast, the game starts the stutter and then I get kicked out to the desktop. I have no clue whats going on. Once in a bluemoon does the computer just restart. Mainly I get kicked to desktop. Iv tried 3 diff Geforce cards all with the same prob. Iv updated every driver in the book that I know of, sound, graphics,bios,ect… I use gamespy lite (for kingpin), gamespy 3d, allseeingeye. Does ne1 have ne clue as to whats going on. I have AMD1800XP,Asus A7V266 (iv tried all bios update), 768DDR,2 45gig hd, XPpro, Geforce2 TI (driver 28.32),Antech 400watt, Sound Blaster Live

Some people have suggested that the prob maybe either the ram ( i dont know how to test the ram) or the mobo. Iv done everything from 2 diff power supplies, reformated 5+ times on 2+ hard drives to ever driver I can get my hands on. Any help would be great.

Are you overclocking?

And if you’ve tried all that stuff you’ve mentioned, I’m inclined to agree that it might be your RAM. Is it 3 sticks of 256megs? Are they all the same type/brand?