[jhirasak]

Faulty power valve diaphragm with a crack or leak which allows the PV to occasionally open to early resulting in over-rich AFR while you are operating on the main circuit. As the diaphragm moves, the amount of the air leak can vary which can allow it to open earlier than desired, hence your sometimes too rich AFR symptom. Each time the engine is stopped, the PV fully opens because there is no manifold vacuum to keep it closed. The next time you start it up, the manifold vacuum should reseat the valve and depending on the amount of air leak through the diaphragm, the PV could open early or stay closed if there is sufficient manifold vacuum.