This has happened to me on quite a few occaisions. I am definately using OpenGL, with the latest detonator drivers for my GF4MX.
I reasently applied the "increase fps" patch that grOOvy suggested and havent had the problem since. However, it could just be a coinsidence - try it out for yourself and see how it goes.
Wat da he said that it also works on cs and other games its just beta 1.1 don't work good then that means your computer don't like it lol im joking.. i still rekon your graphics card because esf takes a bit of a drain !!