What have you been adding water too? The coolant resevoir? You replaced the water pump and the thermostat, but what temperature rating was the thermostat? Is the radiator aluminum or plastic? I know that the E36 radiator is plastic and was prone to failures. Im not sure if the E46 solved this, and i dont know the year of your car. As Big Evil stated it may lie within your radiator, so a good place to start would be to flush your rad out and see if their is any build up or deposits. BMW reccomends only using BMW blue coolant, and i have heard a few stories of people using orange coolant (DexCool) and it clogged the radiator.