Hi Lorenzo,
Did you know, that you need to "capture" the throttle when turning AT off?
To do so do the following: If the throttle setting in the sim is "higher" than the throttle setting on your joystick, you have to move the throttle forward, beyond the actual settings in the sim. This will cause the throttle in the sim to be "captured" and your joystick throttle will work correct from then on.
See this video from Jan for further explanation:
Cheers,
Chris