That is a very old thread. Full names on all posts in the PMDG forums.
Do you realize that 40% N1 or higher is idle depending on speed and altitude? I set the throttle options such that the position is shown when moving, if the N1 needle is at the bottom of the moving throttle blue arc then you are at idle. See also the fuel flow. It will always be about the same at idle regardless of N1... usually 0.82-0.94 lbs x 1000 / hr depending on engine variant.
The aircraft can be slippery and if you are descending above 900-1200 fpm it might not slow down unless there is drag out there, such as flaps, speedbrake or gear. It takes time to learn how to manage your energy, don't blame the aircraft yet.