I recently had what looked like fuel pressure problems, similar to what you outlined here. But worse yet, my '70 GTO was dying when driving around in hot weather.
I had a little autometer fuel pressure gauge under the hood that was also giving me very, very low PSI numbers, like from 1psi to 3 psi vs. 5psi to 7 psi that we need
I thought that my mallory 140 pump failed so I had that rebuilt and then I reinstalled it.
Failed again!
Then, I bought a new Holley HP 150
Long story short, the new HP150 dies on me also! Shortly after that happened, it started working again. What was different ? Heat! I realized that what I was experiencing was likely a form of vapor lock. I am running a factory fuel line and I figured that it was no longer cutting it, even though it worked OK for years.
I took the car out of service, bought a new RobbMc sending unit (half inch), RobbMc regulator (bypass mode) and put in twin -8AN lines, including a return line.
Problem solved. I should have done this years ago
