Hard starting

The truck fires right up when the engine is cold and runs fine. But after driving when the engine is warm (normal operating temperature) and you shut it off, if you go to start it again within five minutes or so it will fire right up. But if you wait much longer than that it is difficult to get it to fire up. Then it takes a prolonged period of cranking it over to get it to fire up and run. (If it is a hot summer day it may not even fire up with prolonged cranking) If you wait for the engine to cool off for say forty five minutes, it will again fire right up just fine. The longer it takes to cool off the longer you have to wait. So on a hot one hundred and five degree day you may set a while. Any thoughts? New good quality plugs, wires, cap, rotor installed, fuel system cleaner used, etc.