I was having this problem tonight, and I think I found a solution. Go to the Arma 2 folder in My Documents, and open *playername*.Arma2OAProfile (you will need NFOPad or some similar program to edit the file). Near the bottom, find the line called "sceneComplexity". I changed the value to 100000, and the problem stopped for me. I'm not sure if this will work for everyone, though, but you could give it a shot.