That's a bit weird
I was having some practice runs in the 111 the other day and didn't have that kind of issue at all. Mind you, my PC is adequate but still far from top notch (i7 920 @ 2.7 GHz, Ati 4890 1GB and just 3GB of RAM).
At this point it could be a lot of things, so i can only tell you what is standard procedure for fixing most of such unexplained "happens to me but not to everyone" bugs.
I would first advise reinstalling the .net and other redistributable libraries that came with the sim (found in steam\steamapps\common\il-2 sturmovik cliffs of dover\redist), install everything except i think the GDF folder as i think it was the initially used DRM that has now been disabled. In any case, even if you install it again i think that it automatically gets deactivated through Steam by running a file integrity check.
Which brings us to the second step, validating your game files through steam.
Finally, before running the sim again, go to documents\1c softclub\il-2 sturmovik cliffs of dover\cache and delete the contents of the cache folder and its subfolder named shaders. This will clear the cache and force the sim to rebuild it. First time you run a mission it will take a bit longer to load as the sim is rebuilding the cache, but then it will be back to normal.
All of the above steps make sure you can get rid of any possibly corrupt files that might have strange effects on the game.
On a side note, before you do all that you could also try enabling automatic updates (if you haven't got them enabled by default) to let Steam download the latest mini-patch (about 20-30 MB or so) if you haven't done so yet.
A lot of people report improved performance with that one, maybe it's an early release primer for the upcoming revised graphics engine that they are about to include in the next patch and it streamlines some things in that regard.
Try this out and let me know how it works out for you.