There is also something here that has not been mentioned and as a 30+ year RC'er I am almost embarrassed to reveal. If your receiver battery pack is low, you would see what you are describing. I was setting up a plane not too long ago and turned on the transmitter and receiver and then set up the servo travel........noting that the throttle servo was adjusted correctly. I turned off the transmitter and the throttle servo went to full open without any input. I turned the transmitter back on and the servo would not center but would jitter and then move back and forth and then make up it's own mind where it wanted to go. Then the rudder started jittering and also the elevator. I thought my whole system had lost it's mind. Then the light came on and I attached a voltage checker to the receiver pack which had been in a drawer for awhile and sure enough my 6V pack had sunk to around 3.4V. I charged it up and everything is fine. If your radio system is new, your batteries could be low so make sure you put a good charge on them and re-check your set-up.

Cheers,
Andy