Rollercoaster ride during landing !!!

I am not sure what I am doing incorrectly but I have made a few flights since owning the 747 and it seems that as I am approaching the threshold for landing the plane reacts violently sise to side and nose diving almost like severe wind shear or a wake vortex.These are on ILS approaches with flaps at 30 and a KIAS of ~ 156.I also noticed lastnight my flaps reseted to up during this strange event.My questions are am I too heavy with fuel (failed to look at my fuel status)for the KIAS,or am I approaching too slow for this size plane.I am using ASV,RC4 and UT.I own other heavies and have not encountered this issue b4.I the wake turbulence simulation on but the increased wake effects off on the ASV options.I also have the latest PMDG update ....ThanksAndy