Slowdowns, Speed Ups etc.

[Skal Tura]

Hardware:
ABIT IN-9 WiMax (orsomething like that, their Flagship with nforce 680)
2xGeForce 8800 Ultra
4Gb DDR800 CL6-6-6-18
Core Extreme 6800 @ ~3.43Ghz

Resolution 1280x1024
Drivers up-to-date, integrated sound. Well cooled.

Windows XP Pro SP2 64-bit (Look for benchmarks, fastest windows for highend currently, beats Vista 64bit by a huge margin of around 20%, XP x86 still with ~15% margin for gaming)

Problem:
Sometimes the game slowdowns, FPS drops down, and right after it speeds up to REALLY fast and FPS is back to normal.

Generally still getting "not enough performance" problems, even i lowered significantly from the defaults, with which worked mostly fine except Le Mans 24hr was impossible -> when it became dark FPS was like 5FPS and my car ran WAY slower than CPU cars in every regard. This was fixed by putting headlight detail to low.

I've also disabled drivers, crowd, motion blur, and still getting these slowdowns.

Sooo ... What's going on here? It's not constant, and does not happen always. Ironically the very first time i played > no any kind of problems, but couple days later, constantly problems.