If players are scrutinized more at the end of games why would people think he performed better in the fourth quarter than the rest of the game? Wouldn't they know he didn't perform better if they are scrutinizing it more? If he performed the same in the fourth quarter, why did he need to have 35 4rth quarter comebacks and 46 game winning drives? I find it hard to believe a legend can be born out of a 70 qb rating at the end of games.
As far as the second paragraph, why would it magic? Have you ever felt stressed in any situation? Why do some people freeze up when they have to give a speach. Others seem to be right at home? Is it magic? I was in the military. They put stressers on you in training to improve your performance in high stress situations. I don't think it was magic that allowed me to perform in situations I would rather not talk about.
Bottom line is everyone performs differently under stress. The amount of stress in the first quarter is different than the last possession of the game and you have to score. Most people don't get significantly better. If the stress is high enough, most get worse. But it most definantly is not magic.
Or, since it isn't magic, they can't perform that way all game.