unfortunately I would have to agree that is ptobably the head gasket. Had exactly the same problem, turned out to be an oil/antifreez mixture in my cooling system. Caused my heater core to rupture, the electric fans did not come on EVER, (cuase there was no fluid flowing in the radiator) when we did finally diagnose the problem had to replace the head gasket, water pump, heater core and managed to save the radiator by manully power flushing it multiple times...