I am also getting loss of framerates. I can be flyin at 12k and whenever I get an airfield in view, my framerate drops to the single digits. Normally, my framerates drop to the teens whenever I'm on the deck near trees, but at 12k, that shouldn't be. I normally get 50 if i'm looking at the horizon.
I have a Geforce4 Ti 4400 Overclocked.
I think it has something to do with airfields.
BTW, if I continue to look at the airfields, the framerates will go back to normally. It's only when I see the airfields at first. If I pan with my trackir, really fast, my view gets stuck for a second, once the airfield appears in view.