I have repaired several joysticks with similar problems. The Potentiometer in that control (throttle) is dirty and dry inside just like a volume control gets in an old stereo. A spray of contact cleaner into the holes in the base of the potentiometer near the shaft and much sliding back and forth of the lever (rotating potentiometer)will cure it. The electrical contact spray cleaner is found at electronic stores. You will need to take the joystick base off to see the potentiometers.