My 1996 Ford Explorer will not start. SOMETIMES?

There's no telling when my explorer wants to start. Usually once a week it just will not start, it will crank and crank but just won't start. I don't understand it. Then sometimes 10 minutes later sometimes 4 hours later it will start right up. It is driving me nuts and i don't want to pay a shop to fix the problem if it's simple.

Could be the starter motor. Need more deets