Another oil pressure problem

Freshly rebuilt 340, not by me, 300 miles on it.
Running 20/50 oil, 65 psi at idle cold, warm up and it drops to 10-12 psi.
Rev to 3000 and 40 psi, pressure goes up with RPM.
If I rev it up and then release the gas pedal pressure momentarily jumps about 5 psi so from 40 to 45 then drops down as engine speed decreases to idle.
Tried new Mopar filter, checked the oil gallery plugs as detailed in previous threads, behind filter adapter, under distributor and above the rear main cap using a rod through the oil pressure port.
This engine was built by a reputable engine builder but unable to contact him.
I am suspecting a weak oil pump, no idea if it was replaced but I would assume it was.
Going to pull the pan and pump this weekend, Any other ideas?
No engine noises, no lifter clicking, have also tried second and third direct reading oil pressure gauges and got the same numbers.