just an idea, you might have an air lock in the system, if you can hear alot of bubbling its more than likely. Run the car from cold with the header tank cap off, then run it all the way up to running temperature. Then switch car off and let it cool with cap off this should release any air in the system. It worked with me after changing my engine, i experienced the same thing and discovered an air lock. If the water bubbles in the tank and bursts out let it do so as this is the air coming out the system, once the air is out it will settle and no more shoul come out the tank.