Slight head gasket blows are hard to find using a gas analyser. If you are using one, the hc's are what to look for. There are also some test strips, that you dip in the coolant, on the market. Have not used these, so can't comment. Still think you have a gasket fault, best done sooner rather than later, over pressure in the cooling system causes lots of damage, heater matrix, radiator, hoses etc, not all will fail now, but in the future?. The head job is not as hard to do as it looks.
