game freeze problems

for some reason after about 20 minutes of play, my game freezes, i run a geforce4mx 420(or 440, me forget) with 256 ram.....on the second highest resolution in opengl mode...i havent had this problem until recently....i havent changed any settings and its now doing it.....somebody help me

do u have enough pc memory and are u running like 6 different things in the background?

i used to have this problem BEFORE i changed to OpenGL, so maybe if you changed to something else it would work?

i have 256 memory and.....well.....maybe that 2nd one....