Why not try doing a Flight level change (press FLTCH button) so that you get a better controlled descent.
Or. Have you tried the DES NOW in the FMS/CDU - VNAV - DES page? Either of those might help with getting a better result.

Other than that, you could try deleting the SimObjects\Airplanes\CS_B757-200 folder, then reinstall selecting the option to repair. This might help sort it out. It really sounds like the installation isn't 100% right.

If you like, you can provide the details of your flight to YPDN, and I'll try it myself and see if I have the same problem.

Mark