Rusty Dusty Clap Box

I have a 1997 Chevy C1500 with the 5.7L. Fuel pressure jumps to like 64 when pump primes then drops, no leaks are anywhere. Brand new pump (4th one in 3 months to test different pumps but same outcome everytime). When truck runs it holds at 55psi but when truck is off pressure drops down to 25-20 psi. I have no idea what it could be besides maybe faulty regulator that could be leaking? Should I pull the trigger and upgrade to the MFI injectors and a new regulator and see where that takes me?