Could be your thermostat. Also, don't use anything other than BMW coolant. YOu should probably flush your radiator. It is a good routine maintenance item that should be done about every two years. Was your coolant boiling? Check your hoses, as maybe you have a leak or something of the sort. I know I am being very general, but I hope at least I can get you looking at the right stuff. I would also urge you to check your drive belt's tension, as it drives the water pump also; however, if this were the case, very likely you would see your lights dim, battery light would come on, etc. Who knows, maybe the pulley driven by the belt is not performing as it should. Good luck.