Also, there's obviously Biden with his stutter, which is definitionally a mental condition as it stems from the neural processing area of the brain that controls speech.
I've suspected for a while that Dubya as well might have a mild aphasia that produces some of the Bushisms. (Dyslexia, as themiddleman suggests, might also explain it.) I wonder if he was less prone to malapropisms before his struggle with alcoholism.
It doesn't show up in writing and thus not in my posts on this forum, but I suffered some blows to the head a few years ago and ever since then I've had a tendency to Bushlike flubbed idioms and mixed-up lexical items when I speak.
