Just a thought: Steady driving under certain conditions can cause the carb to ice up. Long shot.
Ron
When the engine stalls, coast to a safe spot and take the air cleaner off. Is there ice in the venturi? If it's humid might be more of a possability. Feel the outside of the carb, it might be colder than you think. With the engine still off check for fuel by opening the throttle & (safety first!) check for accererator pump spray. Get a vacuum gauge, you can test the pumps ability to pull fuel from the tank & for restriction from the pump back in two simple steps.
