Your problem does not sound like decementing. When that happens it's a sudden event, all you see is the rangefinder patch surrounded by a field of black, and it doesn't return to normal at all. That said, I would send it to DAG. He'll fix it whatever it is, and even if it was decementing (very, very rare in any Leica past the earliest M4 bodies, as later ones--in fact, even the M2R--used light-curing optical cement for the rangefinder prism vs canada balsam) Don will re-cement the prism for less than half what it would cost to replace the rangefinder. This one area IMHO there's a clear reason to choose Don over anyone else in N. America.
