oscillating after VNav decent - mostly

Hello PMDG , allAfter some flights i saw some oscillatingsmostly in VNac decent.This most goes around from 200-500 feetYou see the elevator slowly goes up and down all the timeUsing FSX - Win XP and no weather Engine other than from FSX.And allways the normal weater conditions , no specials.Loading my fuel allways with the loading modul from PMDB optionsand my Payload is set around 80000 lbsHave seen this in Alt Cruise aswell but mutch smaller.I mean the Queen is oscillating allways a little bit up and down.Any one else seeing this issue ??Is this normal ?? ( don`t think it should ..mean the VNav decent part )regardsMario G