Great !! thank you...I "played" with all the sliders as you suggested and I finally got my airplane smooth and almost "real".
But there is another problem: it often looses textures, I mean I see the airplane gray without colors, signs etc. if I switch to external views.
It happens also when the airplane lost textures during the simulation and, when finished, you get back to the main menu where you can see the preview of the airplane.
Even if you choose another airplane and then you choose again the previous, it appears without textures.
Then I have to close FS2004 and then open it again.

Why so?
Is there a setting to fix it?