OK on the adding and then removing extra oil; pretty much eliminates of that question.
What type oil pressure gauge do you have? If mechanical and with a plastic line, make sure the line is not being pinched. If an electrical gurage, like the original Mopar, an erratic connection or ground would raise the circuit resistance and lower the pressure reading. Flexing of the harness and the connection through the bulkhead connection through the firewall and perhaps the ground from head to firewall could do this. Odd and unusual, I know, but you have odd and unusual symptoms....
Have you perchance put on a different gauge and checked? Just throwing out ideas.....I see you tired a different gauge but was it all changed out from the block up or just the indicator?
