Scott,
That was happening to me as well. It was discussed on the IMG forums a while back. I deleetd the game and all the files and then re-installed, and applied all the updates. Works fine now. I actually think it might have been another application that was causing this. I only noticed the lag when I booted the Multiplayer version. I have a little application called Cosmic
Debris which places a little icon in my menubar that alerts me to possible auroras. Just a gut feeling about that. But check to see if you have any small apps using the same port that COD-MP uses.
Mark