my 5 series bmw is overheating but the water in the radiator remains cold until i take off the cap and hot water come in. When i open the cap when cold there are some bubbles coming, head casket? the car is running perfect and there is no sign of water in the oil

As above, if the water in the radiator's cold, then in order of difficulty, it could either be an airlock in the cooling system (squeeze the hoses to see if they feel full of water & not just air), a thermostat stuck closed, or a failed water pump.