andrew66, are you following the correct procedure for turning your model on ? Transmitter on first, followed by the model?
2:- was your transmitter close to something metalic , like your car? This can create a shadow bounce of the signal and will cause your servos to bounce.
3:- Sometimes when your transmitter is lying on the grass, this will cause tremors in the servos as well , if conditions are right, but this rarely happens.
Other than the above, that i have had happen to me in the past , i cant help you . Maybe someone else will have experienced this problem and will be able to help you .
Hope you get it sorted out, and have many happy hours of flying
Bill