I was running it on Vista with a fresh install, but it was very unstable.
FPS kept jumping from 30 to 5 and back to 30 again.
I reinstalled XP again today, did a fresh FSX+SP1 install and now I get a very stable ±30 fps on medium High setting, even without tweaking the FSX config and trees etc.
Now I have to defrag my F drive (where FSX is installed) and do some tweaks...and enjoy the scenery
So it looks like the problem is solved
