I consider myself pretty experienced with complex sims like this, particularly with the FMC and autopilot functions.  I cannot get the VNAV to work correctly on this one.   The VNAV keeps commanding a descent too soon with over a 6000 FPM rate which then causes an overspeed at idle thrust.  Switching to Vertical Speed mode does nothing.  The only way to get out of the excessive descent rate is to shut the autopilot off, re-engage and use VS.  I cannot find any variable to change other than the Cost Index numbers.  Any ideas?

Devan