Very frustrating fuel or motor problem Options

[hars914]

I need help on this problem. It happened out of no where.I have a 71 which just
went thru a 3 year resto. The motor is a 4 cyl. 2.0 built up to 2.4 with weber 40idf carbs and about 1200 miles on the new motor. I was having what seems to be a
fuel problem. I would take the car out and after about 15 or 20 min. the motor would start stumbling like it was not getting fuel. It would get worse then start to stall. I removed the gas tank and it was spotless on the inside,I removed the screen and cleaned it, I made sure the line from the tank to the carbs is clear.
I installed a new Carter pump,I removed the Holley regulator and ran new -6 lines
to the carbs. When I fired it up with the new pump it was pumping about 5 or 6 psi and I noticed it would drop to about 3.5 psi after it would start stumbling.
Ther are no kinked lines.What am I missing here. I also removed the carbs and cleaned out all the jets. Could this be motor related?