1990 ford ranger 2.9L V6 2WD

Truck starts every time and fuel pressure is at 80psi. After a few seconds or sometimes a minute, you can hear the fuel pump start buzzing like crazy, almost exactly sounds like a bumblebee. As the buzzing gets louder, the pressure drops more and more. Sometimes it'll get down to 20psi or 10psi (engine barely running) and then kick right back up to 80psi and run perfectly. Maybe 5-10 seconds later it'll do the same thing, possibly stalling the engine.

There are no leaks in the lines.
The fuel pump has been replaced twice.
I couldn't find anything on the internet about recalled or defective fuel pumps.
It is not blowing black smoke.
You can't see or smell gas anywhere.

The only thing I can think of is a defective fuel pump but 2 new pumps having the exact same problem seems highly unlikely.

Any help would be greatly appreciated. cheers

 

Have you inspected the rest of the fuel pump circuit? Relay could be clicking on and off or the ground could be intermittent or there's another intermittent in the fuel pump circuit.
Fuel pressure jumps to 80 psi? It's supposed to be 40 +/- 5 psi.