Leaking M104 Head gasket
As indicated in other posts here, there is a special version of the gasket and special sealant which are used to cure this problem.
Leaks can also occur around the timing gear cover seal, so do it too. Also, the head bolts should be replaced. It wasn't until all of this was done that the leaks with my '97 M104 were cured - thankfully under warranty. Good luck.