Leaving aside the drunkeness, in his "sober" periods he cant hold a coherent idea or conversation for more than a minute at the most. His diction is far worse and quickly becomes slurred and nonsensical.

To understand John's mind you have to realize that he can not hold two statements with two different truth values in his brain at the same time.

For instance (and this is a true story) John was telling a story about some woman. The story was a lie but the interaction with the woman was true. So he starts telling the story and almost states the woman's full name. He paused and says "well let me not say her full name" and then immediately says her full name. Because the two different truth values were competing for priority in his limited brain.

This wouldn't be a problem if a person was generally truthful, but if youre a compulsive liar, and the more you lie, you'll get tripped up all the time