Hope I am posting this correctly and not reposting someone elses hint.
I have had a few villagers bonked on the head by coconuts and then they get stupid and their name is automatically changed to " ? ". the first wasn't recovering and was just wandering around being stupid. Well after this happened a second time I changed the villagers name to stupid. what do ya know they made a full recovery almost immediatly. So I went and changed the first ones name to stupid1 and it was a miracle.