There is a ceramic block that mounts in your engine fire wall. When these go bad, this is the symptom. The name of it escapes me for some dang reason! I think it is called a thermanl capacitor or something. Sorry for not being professional about it but my mind is blank right this sec. It is usually white and has one wire plugged on one side and one on the other. Secondly and a real big issue on this truck it the fusible links. On these cars they get dry rotted and when the car gets hot, the wire expandes and separates the connectors inside so you can see them. When the van dies again, have some one put the van switch to "start" and hold it there **WHILE IN PARK** and you move the wires back and forth and up and down in a circle motion. Keep this up and try as many wires as you see that have that wierd estra rubber piece at the end of the connection. if it cranks, theres your problem. It will take about 10 minutes to do but I am thinking you have the typical older Ford/Mercury issues. Explorers, Sabels/Taurs', and p/u trucks also have the same issues. Good luck!