Onan Marquis Gold 5500 shuts down after 20 minutes

Another fuel pump check is to get a gas can next to the generator and drop a hose in it connected to the fuel pump and see if it runs better/longer. If it still drops off try putting the gas can up on a chair or table so that it is even with the pump, less work to "lift" the fuel. If that makes a difference I'd double check the fuel line and replace the pump.