R9600 and DOOM3

I am playing Doom3 and it is running smoothly at medium quality however when there is lot of fire and moving objects on the screen the game slows down and even pauses for a second, is it how the card normally runs doom or is there something wrong ?
the system specs:

 

Latitude D800

P-M 2Ghz

512mb

60gb 5400rpm

Ati Radeon 9600 128mb

WUXGA

 and I have the dell drivers , no update