hard start-fuel pressure problem. plz help

98 camaro 5.7 litre. Has hard start problem. Fuel pump been replace 4 times, regulator replaced, filter replaced, relay been replaced. The pressure is at about 45psi when running. Mitchal say 55-60 psi. When i turn the key on pressure builds slow, sometimes to 5 psi, sometimes 10 psi, 15 psi, 20 psi. the car will start at 15- 20 psi. once started will build up to 45 psi, and maintains until i shut it down. The system does not hold any pressure once engine is off. any ideas?