It is caused by the control rods giving or bending under stress. This happens alotwitha single aeliron servo appplications. The faster the plane goes the more stress the control rods are under. Or the clevises are not fitting tight in the horns. This is caused by drilling the holes in the horns to big. Antother reason may be the control surfaces may not be glued or hinged tight or close to the plane. I have had a couple of planes do this also. One is a 8ft Telemaster. It sounds terrible at wot. It is because of the single servo for the aelirons. The wire is just to weak.