I have a Lincoln LS that starts fine when it's cold but will crank for about 10 seconds before starting after it sits for about an hour after driving it. Both pumps have been replaced twice by Firestone. The first time the pumps were replaced the car wouldn't start at all but after the pumps were replaced I have the warm start problem. Firestone replaced them again under warranty but I still have the problem. They referred me to the Lincoln dealership. They mentioned something about re flashing (reprogramming) the PCM?
Your mechanic should take a fuel pressure test not for pressure but for pressure leak down.
When you turn the engine off and wait the few hours the injectors (one or more) may leak and flood the engine. He may need to remove the fuel rail and pressurize the system and watch the injectors to see if they drip. If they do then the faulty injector (s) will need to be replaced.