The way it responds to your re-establishing flow by venting the fuel line to prime the pump plus some time has elapsed since the shutdown makes me think:
Maybe but not so likely, a small air leak between pump and tank.
Possibly something in the tank plugging the pickup causing shutdown but then clearing itself when the flow/suction has stopped for a while.
A flow test immediately at the time of shutdown would be helpful and a pressure test could be useful.
Fuel flow interruption from "vapor lock" will occour in the pump or before the pump---not after the pump because the vapor would be vented through the carb bowl vent.
Carry a helper with you to help in the test, and be prepared to test the situation at the time of shutdown.
Good hunting
