Screen goes gray.

[Agony]

Hi I am new to this game. Well i start the game, it runs at good fps, however ever so often, seems very random, the game freezes for 10 seconds. I have no overclocking. It seems at certain points the game goes gray and either freezes or i have to hit escape and exit out. Does anyone know a fix or is experiencing the same?

Edit: I am on a Geforce4 Ti 4600, P4 2.6 ghz, 768 mb RAM