He had early Parkinsons.
I wonder if he may have had Lewy Body Disease, which can cause Parkinson’s symptoms, and is sometimes misdiagnosed as Parkinson’s.
A link for those who do not know about it;
https://caregiver.org/dementia-lewy-bodies
Parkinsons would have destroyed his great abilities as a stream-of-consciousness comic...how sad. I guess it was just a bridge too far for him. :-(
I watched a short clip of an appearance of Williams on the Craig Ferguson show today...it was wonderful. They are both rapid fire stream-of-consciousness comics and they were at their best.
https://www.youtube.com/watch?v=Yw9Evwb1vtQ
And from a week earlier
https://www.youtube.com/watch?v=wkx4S_MUaG0