I too had a similar problem, however it is difficult to understand why the operation of the multi-port valve would be sensitive to its positioning. At any rate, here's my fix.

Yes the pot became very hot, yet after removing the lid and checking the pot with a straight edge I didn't find any warping. I did replace the gasket, and used plenty of lube. Despite this effort a loss of prime still occured, hence I assessed the intake and discharge fittings.

After draining the pump, and drying the fittings with a hair dryer, I placed pvc primer and glue on the fittings and let it thoroughly dry. I wasn't too hip on glueing the fittings to the pump, however I figured the next fix would be replacing those fittings, that would be an extremely labor intensive process.

Two years later, everything works okay.