The vehicle speed sensor, located on the transmission, sends electrical pulses to the dash instrument cluster for the electronic speedometer stepper motor to function, those pulses are then sent to the ECM, engine control module (computer) so that it can use that data as to determine engine timing and mixture values, as well as to tell the transmission when to shift. It appears that your speedometer being erratic is an indication that the pulses from the VSS are not indicating proper vehicle speed. Changing the sensor is easy. It is located on the back bottom of the transmission, passenger side. Remove the electrical plug, and then the bolt that hold it in, replace with a new unit. If that does not fix the problem, the instrument cluster may have faulty connections, which are not letting the signal get through to the ECM.