Unfortunately, this is happening to a lot of people and as far as I know there is no explaination for it.
The single player is quite a bit older than the multiplayer, however. So there are bound to be various code tweakings that didn't make the cut. It almost seems like the developers spent too much time optimizing the game to run good on lower end computers and none on higher end. Amazingly enough, I run the game at 1024x768 with max settings on a 450mhz GeForce3 TI 200 system just fine.
I'd recommend buying the full version from a place that accepts returns if you are concerned about it.
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