that sounds like the priming of the fuel pump works, but the running of the fuel pump does not. Priming only happened once per key turn on most cars.

Its possibly a few items, like the fuel pump relay. Since the fuel pump works to prime the system, I'd hold off on jumping in to replace that. It could also be the oil pressure sensor, its will cut fuel if it does not read pressure after start up. I would test those two things to see if that solves the problem.

The leak down on the system since it dropped from 50 psi might be indicative of the fuel pump, can you start it? does it idle?