Check to see if your electric fan on the front of the radiator is coming on. Mine just resently Had this problem. I used a better aftermarket one, but you can fine direct replacement fans if yours is toast like mine was. Water pump is a good bet if you are losing fluid under the front center of the motor, then its water pump. Fan is working, no loss in fluid, then look for other small leaks, as these radiators are known for creating a leak on the ds tank, next to the top tranny line. only leaks when hot, and pressure builds up, but when there is loss of fluid there it creats a air bubble, and it overheats due to lack of movement of the coolant. Thermostat can be bad, but if its overheating it has stuck closed, and will cause a fast overheating, due to lack of coolant flow, but the cheapest and easiest of the fixes. Just wanted to mention the electric fan, cause it often gets overlooked as a cause of overheating. but mostly when stop and go trafic, or sitting. hope this helps in some way. These systems are designed so well that it shouldnt overheat. If it does, please find the source of the problem and fix it! Dont try to cover it up, BMW's are not cars that accept any kind of cover up repairs at all. Have someone do a compression test on your motor, there is so many ways that a head gasket can be detected without a serous teardown. Do your homework, then get a second opinion, and please do your ride some good, fix it, it will last longer and wont competely die on you down the road calling on the help of a tow truck.......