I had the exact same problem.
Definitely an air pocket. You might be overflowing and loosing coolant due to bad radiator cap, bad tstat, possible blown hg.
You may be correct in your above post.
The coolant can overflow due to blown hg because the cylinder pressure over-pressurizing the coolant system.Is coolant overflowing when it's cold or hot? Have you replaced the cap yet?
Do you have any other signs of blown hg? Rough idle, mileage decrease, excessive steam and water out of the tail pipe, oil in water or vice versa?