Starting trouble. Help!

[VetteVet]

The two biggest causes of this condition are the NSS and a condition known as heat soak on the starter solenoid. Like Geezer said the neutral start switch might be mis adjusted or just going bad. If it only happens after running for awhile then I suspect heat soak.

You can test the NSS just by pulling the purple wire connector off and jumping between the two wires. A cotter pin works great for this, just don't let it touch any grounds. Just remember that the truck will start in any gear so make sure your in park or neutral or the coil wire is off if you have points and the distributor BAT wire is off if you have HEI ignition.

If the truck starter fails to turn over then you can test for heat soak by jumping from the large cable on the solenoid to the S terminal on the solenoid where the purple wire connects. Same safety precautions because it may start in gear.

This condition is common where headers are used because they allow more heat on the starter than regular exhaust. When you jumped it you put more voltage to the solenoid to overcome the resistance of the heat soaked starter.