Peaceout, if you look at some of his interviews from a few years ago (clips on YouTube etc), he's still an utter arse but he seemed to have a much better grasp of language/vocabulary than now.
From experience with a relative with vascular dementia (cumulative damage from lots of small strokes), people can keep functioning for years fairly well and give the appearance of being ok (most of the time) until they have really very significant damage to the brain because so much of language is repetitive stereotyped responses to common situations (the weather, your new haircut, do you want a cup of tea etc), BUT, take the person out of their usual routine or comfort zone and it starts to fall apart. Trump is way out of his comfort zone now.