2000 Express low fuel pressure

I have a persistent problem with my Express 5.7 1500. Original fuel pump died when 7 years old and was replaced with Airtex unit. Had problems with low fuel pressure and replaced with another Airtex under warranty. Still had poor starting and driveability problems and under advice from forum members, replaced with AC Delco unit. Still had low fuel pressure. At key on, pressure hits 60 psi until pump turns off and pressure jumps back to 50 to 52 psi. This is pretty consistent except at high speed on highway after warmed up where van will intermittently stumble as if it is going to cut off but it keeps going. After three pumps with exactly the same pressure readings (including AC Delco pump) I checked voltage and found 13 volts at pump. Just for good measure I wired power and ground for fuel pump around connector at transmission crossmember. Pinched off return line and pressure hit better than 80 psi. Replaced fuel filter and checked lines for leaks and obstructions and all checked out OK. Checked for leak down and found that presure leaks down slowly (around 10 psi after 30 to 45 minutes and to zero overnight). Replaced fuel regulator (with AC Delco) today and van ran worse. Now very sluggish and fuel pressure is a couple of psi lower. One thing is that when the van is running, the fuel pressure guage needle is not steady but pulsates a few pounds. Fuel guage in dash also has vibrated slightly through all of the pump changes. Could this be a vacuum leak?

Engine cranking or engine running pressure and fuel pump volumn is the key.
First make sure the pump is pumping good pressure and volumn.
A good pump should push 95-105 lbs of direct fuel pressure cold and hot.
If you have good full direct pressure and pressure is droping lower than specs threw fuel pressure regulator than you fuel regulator is bad or a bad leak at injectors.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

Please read my post before copying and pasting the same answer that you always give. Believe me, I have read this exact same reply to everyone that says anything about fuel delivery problems and if you read my post, you will see that all the things that you mention have already been addressed.

I read it and gave a proper test to find the problem.
All free info.
And yes I gave you the same answer as other people.
I would not want to treat you any different.
Sorry.
I can not help you unless you give us proper information.
You can take it or leave.
Did you do the test as outlined?
And post back the fuel pressure that was asked for?
If you did you should have a answer to your problem.
Good Luck