Speech impediments can often take a substantial amount of effort and concentration to combat. They also aren't necessarily something that is overcome and then you are "fixed" and never have to think about again. They can often resurface when tired, stressed, drunk, or otherwise not at peak mental ability. The guy is obviously 12 years older than he was in 2008 and is likely more stressed and working harder as presidential nominee than he did as a nominee for VP. Like I said before, I don't know his mental capabilities and maybe there is some underlying problem there. However lots of the evidence I see can easily be attributed to that previously disclosed condition which by itself provides no indication of a lack of intelligence or coherence.