Try doing this:

Go here http://www.schiratti.com/dowson.html and get the latest version of FSUIPC if you don't already have it.

Go to  the FSX\Modules folder and delete the following:

FSUIPC Documents (the folder and all its contents)
FSUIPC Install (text file)
FSUIPC4.DLL

If you have a registered version, you can leave the KEY (FSUIPC4.reg) file there and get FSUIPC to check current registration to use it when installing FSUIPC 4.XX
If you remove the FSUIPC.reg, you will need to add your registration details again when you get back into FSX.

Those are all the main files needed to have a working FSUIPC. Once they are removed you need to remove the FSUIPC entry in the DLL.xml file.
To access that file, press START then click Run (keyboard shortcut is WindowsKey+R) and enter the following: %APPDATA%\Microsoft\FSX
A folder will open, look for the DLL.xml file make a backup copy of it before you start removing things from within it. You can always delete the messed up one and use the backup so everything in FSX will work.

Remove the following lines:

<Launch.Addon>
     <Name>FSUIPC 4</Name>
     <Disabled>False</Disabled>
     <Path>Modules\FSUIPC4.dll</Path>
</Launch.Addon>

Once done, save the DLL.xml and close it. Reinstall FSUIPC4.
If you are using Vista or Windows 7, make sure you run the file as an administrator. That means extract it from within the ZIP file, right click on "Install FSUIPC4.exe" and select the "Run as administrator" from list that pops up.

Credit goes to firehawk44 in The UNOFFICIAL Captain Sim Support Forum (http://forum.avsim.net/topic/339326-757-rocking-violently-from-side-to-side/ reply #6) in regards to the CS757. Thank go to Michael Cubine (DAL191) who first pointed out the reply in the AVSIM forum.

Who knows? Maybe if you guys do all that, it will fix it for you.

NOTE: I know of one person who has done all of the above "fixes" and his 767 is flying as it should. His words in an email to me: everything works marvelously the plane behaves again correctly and carefree..

Mark