UT stutters

I have xp sp2, and fs9 I recently got UT and really like it but get tons of sutters I only have main roads and landclass right now I dont think framerate is the prob, I have it locked at 28 and it never drops below 20 but usually at 27.9 and steady. What would cause the stutters I heard of something called filemon and .cabs what is this does anyone know?