1980 Chevrolet Caprice starting of car

Good chance the ignition switch or neutral switch are either failing or have been misadjusted. check output from switch with voltmeter next time it happens and also try neutral to start. Both switches are located on top of the steering column under the dash. nuetral switch is pretty easy but ignition switch is a little trickier becase it shares mounting lugs with the high beam switch( both are controled by a rod from wheel end). After this long on the originals you might consider just doing both at the same time as they are not that expensive. While there make sure to check all the connectors for corrosion or signs heating or melting of the terminals and clean or replace any that seem questionable? hope this helps