Had something similar happen in a '96 FLD I was driving years ago. It turned out to be the grounding wire from the cab to the chassis. Might want to take a look at corrosion around something like that.

 

I was talking with another driver with a Prostar too. His occasional no-start problem was a $60 clutch pedal sensor. He keeps a spare now as he had to replace two already.

I misunderstood the first post when I said mine had no click. I was thinking of the click click click sound of a low battery on a car. Mine actually did click but no starter engagement. Turns out the idiot that installed my starter didn't tighten the nut on the positive wire on the solenoid. Amazing it started at all anytime.