Keeping your head stationary during the swing

Club past parallel doesn't mean your not overswinging, you might be pulling your self off the ball with to much lateral motion. I get caught in that.

Other than that, check to make sure your heads not moving in the backswing. You might be lifting up in the backsinwg.

so does tiger. it's not so much a matter of your head moving up or down, it's the moving back and forth that's the problem.

Wow.  Is that true?  I tend to do the same as the OP, moving my head slightly down then up during the down swing, and sometimes down during the end of the back swing then up through the down swing.  I've been told that's something to fix.  Not so?

The body moves in golf.  Since the head is attached to the body, it moves as well.  Forget about it and whoever gave you that dumb advice.  Look at your spine angle and arm and leg motion.  Those are the only parts that matter.

yeah, the body does move, but i'm not sure what that has to do with anything. if the body, and consequently the head moves forward in the downswing, you're effectively moving your swing arc forward. so even if you make a great swing you're going to push the ball to the right.

People are just guessing on this.  I would say you need to post video, both face-on and down-the-line.

It's all a question of degree and ball position.  Pros move their heads slightly forward and down and then slightly back by impact.  It's a response to the proper body motion.  Don't move your head 18 feet forward from the top of the swing.  Letting it move forward a couple of inches is fine.

Lots of fine ball strikers compress into the ground during the downswing. It's a power move and not necessarily a flaw/problem in itself. Watch the white hats on these two guys.