Hi, this is the situation: I load a flight where the NGX is sitting on the runway threshold, ready for takeoff. VC view, TrackIR software running in the background. As soon as the flight is loaded I have about 28-30 fps, that keep quite constant if I don't do anything and don't move my head. 

As soon as I look down towards the yoke and then up again towards the runway, restoring the view that I had at the flight loading, the fps decrease to 21-24 or lower, and start oscillating with negative peaks of 15-16 now and then. They never go back to 28-30, but remain on an average of 20-22.

Is there any particular reason why this happens?

Thank you.

 

More:
I load a flight where the NGX is sitting on the runway threshold, ready for takeoff. VC view, no TrackIR software running. As soon as the flight is loaded I have about 28-30 fps, that keep constant for about 30-40 seconds. Then, WITHOUT I DO ANYTHING, they decrease and continuously change between about 15 and 25. I repeat, I don't do anything, don't even touch the mouse, nothing and the image is static, like a screenshot, with the aircraft aligned on the runway threshold, no AI traffic moving around (AI slider set to 0) and sky clear, not even one cloud.
Why does this happen??? Why don't fps keep constant? What is that causes the fps decrease after that initial period?