Maybe this is not the same case but I used to have similar drop in frame rates occasionally, not frequently. It was really annoying since I do host a lot.
What I found out that my CPU load dropped when those freezes occurred looking like the IL2 was not getting enough time from CPU. Since I have also DualCore system I suspected it was related to core process handling somehow and I decided force IL2 into single core only. That solved my freeze problem perfectly - there is a nice documentation availabe how to do it:
http://il2.netteq.com/Muliti-Core_Guide.aspx
I do not know why this happened in my setup since I know lot of people are running IL2 fine on dualcore systems without any tweaking. Maybe it is related to my MoBo (Asus P5K), but BIOS update didn't help back then.