A couple of things come to mind...1 the servo is indeed going bad.....2 the switch in the airplane could be going bad or 3 the receiver is going bad.....
If it were me...I'd swap out the servo lead for another channel...say rudder for elevator....if the problem goes to the elevator channel then it's not the receiver or the transmitter...it could be the servo or the switch.......change one if that doesn't do it then change the other......if the problem stays with the rudder channel then it could still be the switch or the receiver or transmitter......the cheapest to change is the switch....unless you have a spare receiver laying around.....
Good luck.....