Put a voltmeter on the two input terminals of the starter solenoid, and repeatedly try starting the car. Meter should stay on same polarity for every attempt, and not shift back and forth from minus to plus. If it shifts back and forth, your problem is probably NOT in the so;enoid or starter, but in the ignition wiring, whi h should be very carefully inspected for corrosion, including the ignition switch. Remember this is a 43 year old vehicle, subject to aging, corrosion, etc.
