At the considerable risk of sounding like a broken record, I suggest the following:
1. Set your Head (at Fix) where you want it to be at Impact.
2. Do your dead-level best during the Stroke to keep it there.
If your Head moves...
It moves.
If you can reduce that movement, please...
Do it.
If you cannot...
Then learn to live with it.
And work even harder on your Short Game.
