Intermittent when warm will not start

My truck will run like a champ then it will occasionally not start. Example; run it for thirty minutes to an hour, then park. At times will not start, it will crank but just kind of sputter, I give it gas no effect. Try again and again same thing. Wait thirty minutes to an hour, then it will start right up and run like a champ. I have changed out the engine temperature sensor, new fuel filter and fuel pump Relay. The fuel pump itself is twenty years old. I think my fuel pump is going bad, but mechanics say that it passes the pressure test. Going crazy over here.