I had the same bubbling scenario whenever I topped up the header tank too full. It was cured by rewiring the fan so that it will spin after the engine has stopped. The hot turbo right behind the radiator is a bad combination in a RST after shutting the engine. There was nothing wrong with the headgasket. It's just that when the engine stops, the water flow stops, but several parts still remain very hot for a while and can cook the water.