Being self taught for the most part( only taken 6 lessons) this is my advice. I try to think about holding my chin still and rotating around that point if that make sense.

Try to rotate around your hips keeping your spine angle in tact through the swing.

I had the same problem a few years ago my swing was fine and then out of no where my head started dropping about a foot and a half. I just had my dad hold an alignment stick on my head and would swing. I would focus on keeping my head on the stick so it wouldn't dip.

The number one cause of this is a physical limitation in your body. You are not able to rotate properly because of the lack of movement in you hips and lower back. Get on MYTPI and take a look at some strength exercises to help loosen up your hips, back and shoulders.

A good drill you can do in the off season is to take a small cushion or a pillow and stand next to a wall in your home. Fold your arms across your chest with right hand on left shoulder and left hand on right shoulder and take a address position with the cushion between your head and the wall. Now rotate your shoulders back like you would in your back swing keeping the cushion pressed against the wall with your forehead. Make a down swing and follow through keeping the cushion pinned to the wall. Do this several times a day and then when you can get to the range just remember how your head was stationary when you make your golf swing.