It happened to my 91 300E at 174k miles, the first head gasket was replaced at 55k miles. I tried to flash and change the coolant, but it always come back after a few days. Last month, I removed the head and changed the gasket.

The machine shop charged me $350, which include machine the head and changed the valve guides and seals. The parts cost was about $350, and 5 days of free labor.

I also removed the timing chain cover, and changed both timing chain guide and tensioner.

Jack