nfsu slow down

hi

okay while i'm playing the game a get this kinda lag while in a race.
at first i thought that i might of put my setting to high as a had maxed them out. but when i put them on default i got the same problem.

it's hard to explain but i try.
what happens is that while in a race it will sometimes slow down for a split second and then run normally again and it will do this at least 3 or 4 times in a race. so i was wondering if this is a game issue or if there is a fix for this??
o and i also have the latest patch for the game. i dont really think it has anything to do with my system as i have all the system requirements.
but any suggestion plz

my specs are:
3ghz p4
2x512 mb ddr 466 kingmax 1x256 mb drr 466 kingmax
radeon 9600 xt 256
latest dx
win xp home

i think thats all ya need

thx for any help