Rocking and Rolling on descent

flight from Paris to ZGGG...rollout perfect, climb perfect to altitude...Flight is on course to ZGGG. The FMS is perfect commanding the aircraft...About 150nm out or somewhere at the point of TD. The plane starts having problems with staying on course...The plane has been on course at the correct altitude for 9 hours...When starting to descend from say fl350, the plane starts banking extreme right/left and attempts to stay on course...I have to use manual rudder and steering to keep it on course...I have attempted to use autopilot, decending, but very difficult...I have to fight it all the way to the runway...On the return trip from ZGGG to Paris the same problem happens...Everything is perfect until the TD....except this time I couldn't control the aircraft and it crashed...I am using version Fsuipc version 4 for FSX unregistered (I am wondering if registering would help this problem)2nd REX2 setup on the Server with FSX with the startup program on the 2nd computer using "Simconnect." via the Lan...3rd Using FsCommander version 8.xx setup on the Server computer with FSX using the 2nd monitor to watch the flight...I am flying the PMDG MD-11 instead of the 747-8F this morning to determine if the problem exist with this aircraft...So anyone have any ideas why this is happening? Machine....I7980X with GTX480X video...12GB of DDR3 Ram and Raptor HD using Windows 7 64BitUsing unlimited FPS which usually runs between 50 and 130 at times...However, I noticed on descent that the fps start dropping down to like 20fps...I don't know if setting my fps back to 30 would have an effect on the problem or not...