With some tough old 35mm SLRs I've taken to liberally squirting Ronsonol into the left side of the mirror chamber (facing it) if there are any gaps where it can drain to the shutter arm and linkage, or taking off the base plate and squirting around there. It evaporates quickly enough but you'll see with either of those if it operates better when wet. If it does, then that's a clue to continue the treatment for a while until it eventually runs well when dry.