The boys have hit on the one thing that I completely overlooked. As the fuel is drawn from the tank it must be replaced by air, at atmospheric pressure. I bet the problem is worse when you have a full thank of fuel [small airspace above the fuel] & not quite so frequent when the tank is very low. [large air volume]
When the pressure in the tank is low enough, the atmospheric pressure pushing back down the line from the carb is enough to prevent the fuel pump diaphragm from rising & pushing fuel into the carb.
Think of it as suction from the tank pulling the fuel back.

Diagnosis.
The tank is not breathing, possibly due to either a faulty, or incorrect fuel cap.

It's such an obvious cause & I missed it completely, i'll just go sit in this corner & put my dunce cap on. Durrr.