I'd replace the radiator cap and check all the coolant hoses for flow (as well as the purge-line). Head gasket issues can manifest themselves in different ways depending on the engine. Sometimes the issue will be intermittent until the gasket really starts to go, other times it will go all at once. Sometimes the coolant will leak right into the crankcase, then on other engines it will leak into the combustion chamber and get sucked out with the exhaust gasses.
Interesting thoughts on the carbon build-up issues tho...Be interested to see what other people have to say on that one. Personally i've always ran my cars balls-to-the-wall every month or so (if not every day) to purge excess carbon...All of them ran like new, even with 150,000+ miles.