Have you checked the radiator? When it is hot, not overheated, place your hand on the cooling fins or the radiator. Be careful not to burn yoursefl, but the radiator should be equally hot across the all the flow tubes. If it is not hot or has cold spots, then the water is not flowing thru properly. You may need at have it cleaned out or replaced. If the Rad is hot all the way accross, then you either have a problem in the block or head. It could be a cracked block or head and the combustion gases are entering into the coolant system. There is a check using a reagent liquid to check for combution gases. I suggest finding someone with this device.