He did suffer from doldrums at times and he wrote about them on occasion. But he did manage to work through them. If you look at his poetry from the 60s to the 90s, there are some ups and downs, but The Last Night of the Earth Poems is as strong as many earlier poetry collections. Same with the novels, with the exception of Pulp, which I still find to be a good read. Collectively, Hollywood and Ham on Rye are strong - close to being as strong as the 70s trio. The only genre in which I think he did decline somewhat would be the short story. But that's only because his early works, Confessions, Assholes, Charity Ward, etc. are so damn strong - and real - that his true avenue of experimentation, the 70s-90s short stories, fell a bit short of his previously-set high bar.
My take, for what it's worth.