I'm still leaning something is going on with a head gasket if you are getting bubbles in the coolant. But I'm not an expert on the subject only what has happened to me in the past.
Blown or deteriorated head gaskets account for one of the most common and obvious causes of coolant bubbling. If the head gasket material blows between a water jacket and the inner area next to the cylinder, compression gases will enter the water jacket and send air through the head and into the cooling system. Blown head gaskets produce profuse bubbling inside the radiator and expansion reservoir. Rapid overheating results
I have been there, Never knew I had a problem.
