You might want to really clean the cooling core of the block if it continues...
I have voiced my concerns over this before.... in the old days a rebuild on a block involved taking everything off the block and VATTING it... actually boiling it out with some really strong stuff...
This cleaned up the primary problem area... where the speed of flow slows down at the back of the block and allows the build up of various things in the lower rear part of the water jacket. You might could flush with a 90 degree nozzle placed into the core plugs.. ( MB does not call them freeze plugs )... the use of a vacuum process would be necessary too in my opinion due to the limited size of the drain holes..
This is one drawback of having so many things replaceable on the engine and it being such a long lasting engine in the first place.