Another possiblillity is venturi icing up,,,the faster you move air,,,the colder it gets,,,,when air enters the carburator,,as it drops through the Venturi,, it gains speed and also gets much colder,,,,this could be your problem.
Next time you get off the freeway,,,dont try keep it running,,,just get it stopped safely,,,and get under the hood real fast,,,and get a look down the throat of your carb,,,you may see that the venturi is full of frost,thereby restricting air/fuel flow.