I'm sorry, I can't remember if you tested positive for the AChR antibodies. If not, you should be tested for LEMS (Lambert Eaton Myasthenic Syndrome), because I've read that it sometimes acts this way. I suppose that even if you're AChR-positive, you might as well get tested for LEMS as well--I know that sometimes people have more than one type of antibody. But I think the Mary Walker phenomenon is a good guess, too.

Abby