Pretty much what you think it is or what pro said. The engine will start up and you will drive for a ways until debris starts to build up in either the tank where the flow line is, or by the fuel filters. The longer you drive, the more stuff that gets in there and starts to plug up the line. When you shut it off the stuff goes back the line or away from the outlet in the tank, until you start it back up again. The longer you drive the more its gonna shudder. It could be some water but my best guess is debris from inside the fuel system. Changing the filters on the carb or pulling/dropping your gas tank and cleaning it would likely solve it for now. When you install the new system for more flow just make sure everythings clean and the tank is very clean on the inside. Hope that fixes it. Good luck, have fun.