Sounds like what happened to the k241 in my 108. Took the engine to the dealer to rebuild. The just re-ringed it, said it didn't need boring. Well, it lasted about 20 hours until it behaved just as you described yours. I pulled the piston, had the grey sludge in the crankcase (aluminum) and worn piston skirt. Took it back to the dealer, he had it bored .010 over and rebuilt gratis.
Since that experience, I've rebuilt them myself--less the actual machining. If you want something done right...as the saying goes.
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