well it sounds like my story while back with my car, i've replaced everything about the coolant system and was a waste of money. leak down tested everything that I can think of and .. later on it wasn't a overheating issue, block spun a bearing and finally had a reason to take off the head and it was a small specc in on the headgasket, so yeah sounds like it to me .. somewhere in the combustion camber it's leaking somewhere inside there..