This could just be an issue of trying to think ahead of the conversation.  He's sticking to talking points, the same ones used over and over again by the Republicans.   I wonder if the mistakes are associated with him frantically trying to work his talking points into the conversation and not paying close attention to what he's actually saying.  Either way I find his thinking muddy and uncertain.

There is always the possibility that he's suffering from the effects of age.  My mother started telling me something was wrong with Reagan a full two years before his second term ended.  She would say "I'm telling you, there's something not right with him.  He's getting senile or something".  I've learned to pay attention when she says things like that.  I'll have to ask her if she sees it in McCain.