I like to have a spare servo around just for instances like this. Sometimes this is not an option. I had servos in the past that would jitter; twitch and jerk, but nothing like what you are experiencing. Just to amuse myself when dealing with servo issues, I normally switch the servo's plugs (in my case the ESC and steering servo) on the receiver's channels. I then operate the steering with the trigger of transmitter; control the throttle (ESC) using the transmitter's steering wheel. I do this normally when I have a servo that's not moving at all. In your case I'd say the servo is "toast" by not tracking back to center. If the servo is relatively new, then I'd see about possibly getting a replacement.