Intermittent no crank 91 c1500

[Skywatcher]

When driving truck for long distance or in hot weather, it will not crank. Battery voltage 12.8. New battery. New starter. Cleaned grounds and battery cable. Will always start when jumped. Today, I pierced purple wire (s terminal) near firewall during no crank and was getting around 7.8 volts during couple test. Could it be neutral safety switch or ignition switch? I noticed ignition switch has slots where it can be moved one way or another about a quarter inch. Thanks
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