Fuel pressure problem..I'm at a loss.

I am biased. I dislike front mount mechanical pumps. They amplify any vapor lock proplems because the entire supply path from the tank is under vacuum, therefore boils more easily, 'specially with today's alky polluted fuel

Connector hose sucking shut? Restriction in tank pickup? Pump has a valve problem AKA dirt in one of the pump valves? More debri in regulator? A general restriction anywhere in the line?

Re-re-re check the tank vent system..............................

"After it warms up" might indicate vapor lock/ fuel boiling problem.

If you know what kind of power the thing makes, calculate what full throttle fuel useage should be and do a pump VOLUME test. this is best done by making a restriction/ orifice at the line outlet to maintain fuel pressure, so the pump has to see it's normal working head