I know how to use the autopilot's and how to create a full flight plan with ILS approach.
I program FMS, GTN750, Garmin 1000's etc in Xplane and have no problems.
I'm also a real life pilot so it is not from lack of knowledge that the aircraft have a mind of their own.
I've verified flight models, settings etc.
Maybe it's just the airport of specific fixes that cause the beserk behaviors.