It seems like the tank is unable to hold the pressure from the fuel so there must be a crack or leak somewhere. you may want to inspect the housing on fuel sending pump again. Perhaps a seal or o-ring is not seated correctly since the last time you were in there.
Take a look at our DIY guide on Boxster fuel pump replacement. They mention a the large circular ring with seal that needs to be tightened with a hammer and screwdriver. You may want to double check to make sure that component is snug:
Porsche Boxster Fuel Pump Replacement
-Matt