I believe this problem isn't with the pitch trim, it is with the autopilot pitch control,  which does not work correctly in the current version of the plane, unless you are running FSX Acceleration.   This is a well documented issue, and there is even a download to fix it.  Trouble is, the download is not a proper fix, it will cause the yoke to wildly go back and forth with the autopilot on.   Ther only thing that wroked for me was to purchase Acceleration.  Now it works like it should for me.