I am in exactly in the same boat as you mate and this problem breeds so much unconsistancey!
You find your start swinging properly, taking it not further then 10:30 and keeping the wrist firm, after a few good hits your swing a bit more and the wrists start to collapase etc
I have gone through this so i know how it feels, what i have done is set my washing line up so that if i swing past 10:30 i will touch the line and i know that i've gone to far.
Its amazing the problems overswing can cause. Balance issue, reversve pivot, lose of spine angle yet it does not feel like your doing it!!
Regarding distance, Mizuno is right, i have added so much more distance when i keep my swing controled thats its causing my scores to go bad through new distances with my clubs, On the course using a decent ball and a good contact my 6i is 200 yards where 3 months ago it was 170.
Trouble is i get so excited when i hit the ball good that i start to overswing and go backwards lol and end up getting a

ing off my Instructor!