Here's my opinion on what we're seeing from the Patriots: Tom Brady might be declining some with age, but the big issue is, he's always sucked when he's pressured. Always.
For about fifteen years, fans and announcers alike have watched the Patriots' passing game and made comments like "Brady could have just ordered and eaten a five course meal back there" as he pats the ball about sixty times in the pocket. You give him time, he's as good as anybody who has ever played the game. You pressure him up the middle, and Dolphins fans of all people should know this, he can struggle so badly that he starts having a tantrum on the sidelines. Sometimes he even gets benched (it's happened against us, folks).
This is what happens when you start losing OL talent, and your OL coach retires, and you trade your all-pro guard on the eve of the season for a third string tight end from a bad team.