sounds like you have dynamics problems.
You can try making changes in the .air file and the aircraft cfg file but as what to change is trial and error.
I would suggest that you find a plane that is very similar
to this one and compare the aircraft cfg files and the .air files. I must warn you this can be a time consuming efort with no guarntee that it will solve the problem.
If the model file is bad you are stuck. I suggest you try to find the plane from another source.
