Your system should be able to run dod pretty damn well. not too sure why youre running into problems.
im just gonna throw some ideas out there in no particular order
make sure AA is off and your ingame refresh rate is matching your desktop refresh rate. try newer/older dets. i know it sounds stupid but some people swear that different versions give them better performance. try different sound drivers. get 3dmark and run some tests and see where your system is compared to others like yours. does this happen only online? if the server is chugging or your connect sucks (packetloss, unsteady ping) youre gonna catch lag that feels like your fps are chugging. make sure nothing is running in the background that you may not know about. get ad aware or a program like it and do a scan. defrag your harddrive. format. hehe gl hf