A while back I had similar problems, and tracked it down to be sound related. I see that you say it is not related to radio speech, so it may not be the same problem, but maybe you could try to lower the audio settings just as a test.
The test I used was the track called "F4F vs G4M". When the planes are taking off from the carrier, I always got some freezes, even if I lowered my graphics settings quite a bit. I then realized that the freezes occured everytime an explosion sound was to be played. When I reduced the audio settings the freezes was gone.
At the time I was using a Soundblaster Live! card. I do now have a X-Fi sound card, and have maxed the sound settings again, with no freezes.
