Mechanic problem. 1988 ford bronco?

I have an electrical problem. I have a great battery and alternator. But if i drive, turn off the vehicle go inside the store. Grab a drink, come back and try to start it. Its like the battery is dead. I have to wait 15 minutes or more for it starts again. Its a 1988 ford bronco full size 5.8l

All anyone on YA can do is guess; you need to have a mechanic do a diagnosis… There's many possible reasons

Batteries don't miraculously recharge themselves from dead - remove, clean and tighten the battery cables (BOTH ENDS), and try again. If that doesn't do it you will need a multi-meter and try to find out where the problem is starting (switch, relay, starter etc).

Could be a bad starter. Do the lights/radio/windows work? If so, it OBVIOUSLY is not like a dead battery. It also could be a bad solenoid. I'm guessing when the starter or solenoid is hot, it doesn't work.

Bad starter bad wire to starter bad wires to starter soleniod even the little ones

May be a bad head gasket. It leaks water into the pistons, causing a compression lock. Once it cools down and the steam condenses back to water you can start it. If you constantly have to fill the radiator and your exhaust seems to have a lot of steam or water coming out, fix the gasket.