2002 Chevrolet Silverado

Engine Mechanical problem
2002 Chevy Silverado 6 cyl Two Wheel Drive Automatic 95000 miles

I have a 2002 Silverado 4.3L 95000 miles thats not holding fuel pressure and I cant find why? I have replaced the filter, pump, regulator and injector o-rings. When I first turn the key the pressure shoots up to 60psi then as soon as the pump cuts off it falls to around 25psi and holds there. It will run if I crank it around 50psi but misfires and gives the P0300 reading. So what am I missing? I thought it was a bad fuel pressure regulator to start with but that didnt solve it.I even replaced it again. I plugged the return line and the pressure held around 55psi and it ran fine. What else could it be? Frustrated! Ugh! Lol thanks guys for your help
July 21, 2010.

I need you to run a check. Turn the key on and allow pressure to max on the gauge. Make sure all air is released from the gauge. Slowly pinch off the fuel supply hose. Shut the key and see if it holds pressure. If it does, the problem is before the fuel rail. If it still looses pressure, check for leaking injectors, or there is a problem with the fuel pump sending unit.

Let me know what you find.

It holds pressure when you pinch off the return line. Already checked the injectors and I replaced the fuel pump too

Jul 22, 2010.
If it holds pressure when you pinch the line, the problem is before that. If there are no leaks, there has to be a problem with the sending unit. Have you checked that?