I saw this clip, or one so strikingly similar I can't remember the difference, about a year ago. It's a back-to-back comparison of Bush speaking ten years ago during the TX gubernatorial debate and some more recent clips of him speaking, including during the Bush-Gore Presidential debate.
The difference is absolutely night and day. Frighteningly so. The clip and the linked blog entry suggest it's pre-senile dementia. I think it's a bit of a stretch to diagnose him from this sort of thing, probably put together with an agenda, but what sort of other explanations are there? No, really, I'm asking. Better preparation? He hasn't spoken well in any speech I've heard him give. Is he being coached to speak slower? That doesn't explain him stumbling over his words so much. Stress? Lack of sleep?
I don't think we really have any protection against a President that goes senile or suffers from Alzheimer's while in office. There is a clear line of succession if a President dies, but not if his mental acumen degrades or becomes impaired.
Could this explain why Dubya seems to only speak in front of pre-selected, Republican audiences? Any flaws would likely be forgiven and considered unimportant by those already of the faith, rather than those looking at him with a skeptical eye.