Drop in Frames at altitude only

I have noticed recently that, while getting really good frame rates on the ground and up to about 17000 feet, including detailed payware scenery like Copenhagen (typically 20+ FPS and smooth) as soon as I hit the threshold of about 17000 feet my FPS drops dramatically to 9 or 10 from inside and outside views which is reversible upon descending below 17000 feet? Any ideas welcome as I am not sure where to go after trying disabling ASN,  Ultimate Traffic 2, changing aircraft type all with no impact on this. Thanks

DaveGi

Whoops sorry everyone for the double entry on this topic, I pressed than darn Post button twice believing it was not doing anything. Apologies.