I think you mean it has an oscillation not flutter. Flutter is the response to an out side force which can set up a self sustaining movement of a surface. If the servo is oscillating and then stopping after being exercised a bit there could be several causes. Some friction in the devices being moved causing it or it could be that the servo has a very tight dead band set up (which is not normally a bad thing) or, highly unlikely, a dirty or worn feedback potentiometer inside the servo. Do you also have the nose gear steering connected? If so, any friction or uneven strain by the nose gear could be causing this.