BMW 318i water boils over!

[BACR]

Car: 318i 1996

From about 1000 miles ago the water has been boiling over the radiator filler cap every time i stop the engine of the car. I took the car to bmw and they said it was the radiator that needed replacing.

I replaced the radiator today but it has the same problem, it boiled over after you stop the car.

2 garages have said its not the head gasket. And about 2000 miles ago I had the thermostat changed as it was giving incorrect readouts (1 miute it would be fine then it would say the engine was dead cold).

Please help!