after bypassing t-stat, and still overheats. It could be as witeshark suggests; a fan clutch; or a clogged radiator.
Overheating is a sign of head gasket damage-that is true. but unless the car:
2)has oil in the radiator, water in the oil;
3) or, you have steam or water coming out of the tailpipe;
then it's possibly just some prob with the cooling system.
*then, a lot of owner's just do not know that BMW cooling systems require that the cooling system be bled and purged of air-thus, the car overheats.