hi
i have a friend that when he gets near the end of his round his game fall to bits at times, we noticed thet when he gets a bit weary, his left foot come up onto his toe on the backswing and he sways back a lot more, then swings down out to in and often hits behind the ball, now he keeps his left foot on the groung, he still does it now and then when he is tierd but he knows whats doing it.
check your not doing the same, also take a video clip and put it on here and some of the guys on here will be able to help you more.
hope this helps a bit
bill