It used to happen to me, too. I did a few things, but don't know what ultimately fixed the problem.
I turned off anti-aliasing, updated my video drivers, and increased the size of my swap file. Now my swap file starts at 1.5 GB instead of 768 MB and maxes out at 2 GB instead of 1.5 GB. The game generally caused my swap file to peak at 1.1 to 1.2 GB anyway, so I figured that I might as well have it set to above that by default.
As I said, though: I don't know exactly what solved the problem, but I don't have it anymore.