You can't disconnect the servo motor from the gear train and expect anything but bizarre behavior.

The whole point of a servo is to run the output gear to a specific position based on your control input. That position is sensed by a potentiometer, usually connected directly to the output gear. The motor runs the gear train until the potentiometer reaches the desired position. If you disconnect the motor from the gears it can't move the output potentiometer and will just run at full speed. It will probably change direction (CW or CCW) as you move the control stick.

Look at those gears more closely! I bet you have missing or damaged teeth somewhere in the gear train. You can buy replacement gears for most servos. Check on the Tower Hobbies web site by searching for "gear set". You might be surprised at how many sets are available!