stutters

hi all, doom 3 is giving me a problem. My pc config is: Amd athlon64 2800 512 mb ddr ram geforce 6600gt 128 mb 160gb pata hdd while running doom3, it stutters many times, mostly when im in front of a door, it pauses or stutters for 2 or 3 sec and then the door opens. it also happens...