Best Mechanical Fuel Pump

This IS odd.

As much trouble as you are having you should eventually see "junk" if that is the problem.

But the odd thing is that WOT seems to trigger it. Mechanical pumps "pump" by means of the spring in the pump. That is, the cam "cocks" the pump arm, and the actual "pump" is when the spring pushes the arm back down. What this means is that WOT should NOT affect pump pressure.

Regardless, my next step would be to "rig" a pressure gauge and see what you actually have.