If the servo goes right & left when you operate it ,then that is right ,but if it goes haywire ,then something must
be shorting out ,it is possible that there is a factory defect in something!
That is probably why no one has said anything an dont know why!
Usually ,that will happen if there is a short in a receiver or a fraid servo wire ,also ,if the servo is shorting out
due to moister or wetness inside!