FPS drop at a certain flightlevel

Anyone else who got this problem when the aircraft is passing FL160 and up, the fps drops dramaticly from 50-60 fps down to 15-20 fps until descend and under FL160 everything suddenly go back to high fps just like that? I've reinstalled the FA50, tried with older Nvidia drivers but nothing helps and this problem started recently. The Cessna 560XLS had the same problem but it went better after reinstalling. I use RXP GTN on both aircrafts. Other planes like the PMDG 737 or Aerosoft Airbus series doesn't act like that when flying. Even the Beech Premier stays ok with the RXP GTN addon during the whole flight.