Q: Starter solenoid - 1986 Ford F-150

The starter solenoid is most likely not the problem here. You most likely have a loose or bad cable. I would first recommend using a volt meter to see if you are getting at least 12 volts on the battery terminals. Then measure voltage at the starter solenoid to see if you are getting the same voltage or the cable between the battery and solenoid is losing connection.

If the voltage is good then measure voltage to the small terminal on relay when someone pushes in the clutch and tries to crank the engine. If you do not get power then you have a problem with the clutch switch on the clutch pedal linkage.

If you have vehicle in neutral with key off you can jump the small relay terminal to the battery side terminal and the relay turns then the starter relay is not the problem but the small signal wire is not getting power from ignition and clutch.

To avoid any unnecessary repairs, have the no crank condition diagnosed by a mechanic like one from YourMechanic if you need someone to properly inspect the problem of why the starter solenoid keeps going bad.