Ck your Batteries, if using alkaline low batteries, NiMh it'll quit completely, Lipos ran them down slow low you cant recharge them.
Both servos at one time, I don't see it happening. Removing one servo and other one quiets down cuz servo takes power, Or remove both Servos and insert the throttle servo to steering, if ok, chances are the servo is fine, then remove from RX. insert the Steerin servo into steering TX if it chatters Servo is flakey.