V770 non ultra, and OpenGL games. HL Q3

I am having the weirdest problems with this setup. If I try to play Quake II, it runs just fine, but as soon as I move the mouse, the game crashes. When I run Quake 3. I get to play for about three minutes, and then it crashes. Half-Life does the same thing. Frame rates and resolution kick ass for the three minutes that I play! but I don’t know where to go with this. I have an Abit BF6 with an overclocked celeron 500 @ 564 Mhz. 256mb ram, Vortex2 Audio, Supra express modem, and I have installed a Fan on the heat sink of my non ultra V770. If anyone has an idea, please E-mail me at twist118@earthlink.net thank you