I think this is what's been causing issues lately. His tolerance rises since it's require in such high doses to keep him looking somewhat alive, but it's reaching a point where the dose isn't enough to wake him up while being so much that he's clearly tweaking.
Those first debates made it obvious. He's all spry and chipper for 20 minutes, and then suddenly his voice goes out, he slows down, and he looks confused as fuck. You can hear it in his voice. There's usually a minute-long stretch where he goes from regular old man voice to that wheezing-through-his-words sound.