1991 GMC Sonoma intermittent starting trouble

Welcome to the forum, I'd have to make sure the battery has a full charge and the connections are good. Also, when the problem occurs, With key in the start position, I'd check for voltage at the starter solenoid, purple wire. If no voltage, I'd backtrack the circuit. Possibilities, ignition switch-- feed to or from the ignition switch-- neutral switch-- wiring circuit. If that wire is hot, check the starter field terminal for voltage, if no voltage, I suspect the solenoid. Most parts stores sell the solenoid and starter as a unit. You used to be able to buy the solenoid separate, maybe still can. I usually just the unit. Any testing at the starter, make sure the tranny is in park or neutral and the e-brake is set.