Low FPS at high altitude

Hi all,

I spend some of my time flying around the world in the NGX, together with fs2crew and opusFSX. My performance is good on the ground, but as I climb to cruise altitude (around flight level 300), the FPS drops to around eight (from 25-30 on the ground). I have set up OpusFSX with the lowest ground visibility (32 km), and also reduced high altitude visibility (which should be 96 km according to the manual). Does anyone have any ideas for why my FPS drops? If I change the view to only be of the virtual cockpit I get back into my locked 25 FPS.

For the record, I'm flying in P3D, but I do not think this is the issue, rather some of my graphics settings/high altitude visibility. I used to fly with ActiveSky before, and I did not have this problem at high altitude.

Appreciate the help.