You might have a radio interference problem or glitch. If you turn on the car and the remote right now, will the servo work at all? or does the servo no longer respond. Try to field test the radio with the car not running. If the problem does not happen have a second person hold the car in position while the car is running and walk away with the remote and see if it tries to take off on its own. If it does make sure you have fresh batteries in the radio and rec, the radio wires in the radio box (servo wires & battery wires) on the car are not crossing or wrapped around the antenna wire of the receiver.