i own a indy bmw shop, and i can tell u the problem is in the block, the block is alum and when the first head overheated it pulled on the head bolts which pulled on the threads in the alum block, they might be able to get the head bolts retorque to specs, but after awhile they will loosen up because the threads in the block are stressed out and can't hold the pressure, that's y it will leak after awhile, they need to install time-serts(helicoils) in the block, or replace the block, most dealers replace the engine for this reason, each time they retorque the block it gets weaker