I've got a theory that probably just a bunch of malarkey. What I think is happening with Joe is that after finishing his second term as a Vice president he thought about retiring. He probably used to do all sort of daily verbal, mental and visualisation exercises to reduce his stuttering for the past fifty or sixty years of his life. Naturally once he has stopped his speech deteriorated. Poor Joe was dragged by Democratic party to run for office. There is a chance his speech will return to 2016 level before the Trump debates if he returns to rigorous daily exercises.