Vapour Lock Issue

60 mph cruise, cough and die, does not sound like vapor loc. Nor does it sound like a gradual loss of fuel delivery.
At this speed and throttle setting, the carb is running on the low-speed circuit. I agree that the cough may indicate a very lean condition. Since it's ongoing, and you didn't mention a hard restart with long cranking times, I would suspect a problem in that circuit; possibly an intermittent obstruction in the idle well. If you can't find it there, I would check the return line for air bubbles, which would point to air being sucked up by the pump somewhere.But I really don't think it's air. Water?maybe.