Fueling & Injection - fuel pressure issues

Ok I bought my 98 camaro z28 used with 40,000 miles on it. This car is a freaking lemon. I have replaced the fuel pump 4 times with stock gm parts from the dealer. I put in a new pump and i have good fuel pressure at 55 psi for 4 to 5 days and the car starts up fine. then after that when I turn the car to the on position the fuel pressure goes up to 55 psi then immediately drops to zero with in a few seconds. I have swapped out all the relays. What I want to know is what is causing that?
and is there a vacuum line that connects to the fuel pressure regulator in the tank? If so that is missing and could be my problem...

If the pressure falls to zero, you have one of these problems.

1. An external leak, which you would see....
2. An internal leak, which would not be duplicated with 4 new GM pumps as they are fully assembled.
3. A bad regulator, which comes with a GM pump so you should have had a few different ones in there already.
4. A leaking injector, which you should check.

Not sure why it is good for a few days????