The thermostat usually gives warning of lower running temperature or poor heater output if it fails open and over heating if it fails shut. I would change the thermostat next cooling system flush if it has not been changed in the last 6 years, 60,000 miles. Waterpumps on some cars in particular are an issue or common failure. Some VW, Audi,Jaguar and BMW have the water pump impeller fall off. The water pump is not really a maintenance item . Call a local independent BMW shop in your area and just ask them do they see a "pattern failure " of water pump issues with your model year and see if they recommend replacement.