I had something similar to this happen with a JR quattro system that i used. Only difference was all servos would do it. I changed recievers (used a friends reciever and radio) and i never had a problem. The LHS tested a new reciever, and the problem never happened again, till about 6 flights later, and I lost my plane. If i were you, i would get the reciever tested, and not use the servo again. Its not worth losing an entire plane.