Has asthma been ruled out?

Personally, I usually wheeze -- but, have had the coughing spells. My mom's asthma is almost totally coughing -- and it sounds a lot like what you describe -- sudden, non-productive -- and hers is very LOUD.

Of course if you have bulbar MG symptoms, you could be coughing because something doesn't feel "right" in your throat. I've had that happen, too, during exacerbations -- usually after I eat.