On Most Bmws there is a sensor that goes in the radiator, (i beleve) that lets the Aux. fan know when to kick on. I have known a few of these guys to go out pretty common problem after 10 yrs of service. This is only the case when sitting still. When moving and it over heats it could be a handfull of things, From stopped up catalytic converter (burning excess oil can cause this which in turn cane cause the head gasket to blow)) to just needing to flush the radiator. I would flush the radiator anyways. IT also could be vapor locked from running it low on coolant. to check if vapor locked, pump the top hose to see if it has air or water in it. (air=compressable, water=non compressible)