Q. My plane is fluttering rapidly in flight, sometimes becoming uncontrollable.
A. What you are experiencing is too low a resolution for flight model calculations. By default, X-Plane has a 1:1 ratio between rendered frames and calculated flight models. In some planes, this default resolution is not enough. This results in over-compensation which manifests itself in "fluttering".
This can easily be corrected by adding an extra flight model calculation or two between every frame. To do this, simply go to X-Plane's "Settings>Operations & Warnings " menu and raise the "flight models per frame" to 2 or 3. That should fix the issues you're seeing.