I just tested the Duchess in X-Plane 10.11 and 10.20, and I see nothing wrong with it at all. When trimmed, it flies perfectly steady. Autopilot is also working correctly. If you want any kind of assistance, first, I recommend you show a little more tact. Coming in here and making outrageous opinions like "it flies horribly", is not the best way to go about getting any kind of help. State the issue and provide as much detail as possible. Opinions, literally, do nothing to help me identify the problem.