"First follow Rollie's adv
"First follow Rollie's advice. Check that you are not building a vacuum on the fuel tank. Try opening the gas cap while under way and see if the problem goes away. (Clogged breather vent.)Then check to see whether your pump up bulb is flattening out. (Clogged pick up tube.) If neither work. Route a clear plastic fuel line from the pump up bulb to the low pressure fuel filter to see if you are getting any air bubbles in the line as you are underway. If all that fails, I would go back to check the fuel filters and make sure they are clean, and seated correctly. Also, after you experience the slow down, pull and check your spark plugs to make sure they are all clean, dry, and have a minimum of carbon build up."