my car is over heating. water and coolant evaporating at blink of an eye

my car is overheating. there are no leaks. had a pressure test done. there is no water mixed in with the oil. changed thermostat, water pump, radiator hose. service engine light is not on. went thru 3 containers of coolant within an hour. can pour gallons an gallons of water into the water tank and it jus keeps disappearing at the blink of an eye. there is nothin leaking out. so where is the water and coolant going? took car to mechanic shop. they did not see anything wrong and do not know why it's running hot. when open the radiator cap water pours out of the over flow. bled the car for air.there was a lot of air that came out but then the water started pouring out consistently. car can run for hours in park without getting hot. but gets hot while in driving on the road. it gets all the way to the red. what can be making the car run hot? where is all the water and coolant going? and how come mechanics cannot find anything?