Good shot there, TEd. I'm sure I have heard a Yiddish word for what we're looking for, but it isn't coming to me. Need to consult some Yiddish dictionaries or phrase-books. The only English words that come to mind are 'party pooper', 'spoilsport' and 'marfeast', none of which is really it.

In my youth, someone who was always stirring up trouble just for the fun of it was called an 'agitator', but this implies some degree of maleficence, although some agitators I have known did it as a sort of practical joke, like a lot of practical jokesters who have no common sense and really hurt people.