Sounds like the fuel pump is loseing its prime. To test it just screw a fuel preasure gauge on the test port and see if it still has preasure after you shut the car off. It should hold preasure for a long time, as in days after the car is shut off. If preasure goes down it will take a little time for it to pump back up after it drains down. Another less acuratre but easier test is to wait till you know it will have a long crank time and try turning the key to the run position for about 2 seconds before cranking, this will allow the pump to build the preasure back up and if it fires right up after letting the pump run a couple seconds first you know your pump is in the early stages of dying.