The old man's story was not
coherent; he rambled on about several irrelevant subjects.
Her poetry is filled with beautiful language and haunting imagery, but there are no
coherent themes or ideas to bind the words together.
Alcoholics can often be perfectly
coherent even when their blood alcohol level is more than twice the legal limit, since their bodies have built up a tolerance to the alcohol.
I think she has been drinking because she can't even formulate a coherent sentence.