While I'm in flight the plane will start shaking violently and it's speed will drop just above stalling speed. Does any body else have this problem and does anyone no how to fix it. I recognized that it tends to do it most when REX is converting the weather files. I don't know if that's the problem or not.

What's the outside air temperature during this episode? If you are at FL300 or higher and the temperature is not in the negitaves (Celcius) then there's your problem.

When you use this type of aircraft this issue can happen frequently if you don't set properly FSX. first of all disable fsx turbulence inside fsx menu options.

 

Then

 

Are you using FSUIPC?

 

I saw you used rex for weather, but there is a well known problem, and solved, using ASE and pmdg MD11 or 747.

 

In ASE forum there is a 'what to do' for this issue:

 

http://support.hifitechinc.com/Knowledgebase/Article/View/11/4/how-to-prevent-s-turn-issues-with-certain-aircraft-autopilots

Most of the time the temperature is between -17 and-22. It's always in the negatives.

Thanks I'll change some stuff in the fsx.cfg like it said in the forum. And use ASE with REX. Hopefully that should help.