Have you got a good battery or checked its output, if it is the original BMW battery it will be starting to fail. If so this can cause strange electrical problems even though it may still be able to start the car ok.
I believe there is a secondary instrument control module built into the speedo assembly which could also be at fault?
Before replacing the BC1 check it for damp and corrosion, especially all the wiring connectors as they can sometimes be dried out, cleaned up and re-used. Also check it has the later plastic water shield fitted. More here: The BC1 (Body Control Unit) - Forums