/6 sputters under load

If it's missing under load, that is probably an electrical problem with the ignition system. If it's surging and acting like it's running out of gas, it's likely a weak fuel pump or partially plugged fuel line. A fuel pump volume test would tell pretty quick if that is the problem. If the supply is plugged, don't forget to check the sender tube and filter sock. Check to see that the fuel hoses from front to rear are in good shape. A crack on the supply side of the pump will allow air to be sucked in the line instead of fuel from the tank.