I would guess that Joseph wouldn't dip due to the huge number of other famous Josephs. (St. Joseph being the most famous). http://en.wikipedia.org/wiki/Joseph_%28name%29#People_known_...
Compared to Winston (http://en.wikipedia.org/wiki/Winston_%28name%29) which is only common in British/American history.
Is there a name that experienced a dip, and then became popular again, due to a significant person?