2001 Ford Windstar Cranks but won't start

I drive a 2001 Winstar Van sometimes not always when I go to start it it cranks but doesn't turn over and sort of sputers when you start it for instance I drive it for say an hour works fine started right up and then if I was to stop and shut it off and go back out right away it would crank like it was gonna start but it wonts turn over..... then if I waited an hour or two it would start right up can you tell me what the problem is...??