If we're talking "noticeable" as far as drinking goes I will never forget him slurring "Words can't say what love me do" in Montreal summer of 2006. But that could have also been a mix of the painkillers too.

But like everyone is saying if he's been drinking since the 80's something would have changed, he was competent and pretty eloquent in interviews until the last decade or so. I just don't think it's booze and booze alone.