Q&A

 Master

I remember some BMW, VW, and Jaguars around this time used water pumps that had plastic impellers that failed and did exactly what you say. I would call an independent BMW specialty shop ask them if they have seen water pump impeller failure on your year make and model, call several shops they don't even have to be in your area. perhaps Google BMW "water pump failure" and see if you get any results.
With thermostat out and water hoses at radiator disconnected water put in the suction side and engine started water should be pumped out the out let hose increasing with engine speed.