Call of Duty 4 Delayed Performance Lag

Dear OCF:

I apologize in advance for creating an account to ask this question, I realize it is selfish. Though I would extremely appreciate a helping hand. No other forum thus far could help me with this problem.

I installed Call of Duty 4 and patched to the latest update. When I start the game in Single Player, I can play it with the absolute highest settings and pull 80 fps, absolutely no lag.

I can also play with max settings in multiplayer for a couple minutes, but after about 5 minutes or so the game gets steadily more and more laggy, and after about 10 minutes it becomes unplayable (talking like 5 fps). I have never experienced lag that has delayed response like this, it is very strange.

That said, it only makes sense that it is network lag, but it does not seem so as my ping is <40 and I am using a wired connection.

My specs:
Intel E4300 (horrible Core 2, bottlenecks my system)
nVidia GTX275
4 GB RAM


I would highly appreciate a response, thanks for even reading this far.

Are the temperatures alright? Both CPU and graphics card? The fact that it takes a few minutes seems like something is starting to throttle back. I know it would happen in single player also, but can't hurt to check.

What maps are these are, because theres a couple that have issues for 20% of players where it'll lag in a certain part of the map.