What's wrong with John McCain?

Joe posted a video yesterday from Jed, showing McCain trying to answer a question from an audience member, and the video is really disturbing. From three different angles you just watch McCain get sort of totally confused. It's like one minute he's there and the next he's not. McCain's campaign would like nothing better than to demonize anyone who dares mention McCain's age (he's going to be 72 this month). But the fact remains that there's a real question as to why the John McCain we knew in 2000 was vibrant and on the ball, and why John McCain circa 2008 seems prone to repeated gaffs and bouts of confusion. And let's not forgot, McCain got tons of media attention in 2000 as well - and he wasn't at all as "off" as he is today, so it's not just a question of "oh, candidates make gaffs when they're on cam 24/7" - McCain didn't have these problems in 2000.