I think that you have a beautiful swing, and I agree with PerfectImpact that you shouldn't change it if it works well.
I think that I can understand why your head drops during the downswing. I think that it is due to your tendency to drive your right shoulder down the RSP line at the start of the downswing.
See -
http://perfectgolfswingreview.net/Tongzilla.jpg
I have placed a red line at the top of your head at the end-backswing position, a green line over the likely position of the C7 vertebra, and a yellow line horizontally in line with your left shoulder. In images 2 and 3, you can see how you drive your right shoulder downwards (below the yellow line) and that causes your upper thoracic spine to tilt downwards in the direction of the ground, and that causes your head and C7 to drop.
Note in images 4 and 5, that Sergio Garcia does exactly the same thing (it is easier to note how his upper thoracic spine is tilting groundwards in the late downswing).
Brian can comment as to whether this "upper thoracic spine tilting phenomenon" is a fault. I always thought of it as being perfectly acceptable, and I have personally tried to drive my right shoulder more aggressively downwards in the direction of the ball at the start of the downswing.
Jeff.