90 Celebrity Wagon - fuel problem?

1990 Chevy Celebrity Wagon, 3.1 engine, MPFI
I'm stuck trying to figure this out. Here's a little history. Sorry, it's a little long, but I want to give you all the info I have.
I started to lose most of my power intermitently while driving. Not a gradual loss, but sudden, and then a sudden return to normal performance.
The pressure seemed low at the fuel rail, so I replaced the fuel pump, strainer, pulsator & filter. Still low power off and on.
The pump is getting power with ignition on, so its not the fuse, and I don't think its the relay. Oil pressure is good, so the backup oil switch should be working anyway.
Went out and got a pressure gauge. The pressure at pump prime is a little over 40psi, so that is right, but immediately after 2 second prime, pressure drops fast to about 20psi. I tried to pinch off first the supply line, and then both lines to stop pressure drop, but no change. I'm not sure I was able to completely pinch the lines during this test though...
Sound like a bad regulator letting fuel back to the tank? Maybe bad injector(s)? What is the best way to proceed?
Tried to pull codes, but nothing came up. Any help would be greatly appriciated.
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