Thanks very much for the advice. I think I'll start by pulling the sideplate and cleaning since I know I can do that and it doesn't cost anything. I looked at the discussion on fitting a new cylinder stop in the Kuhnhausen book (maybe should have done that first) and it certainly is behaving as though the bevel point on the stop is too short, which means a replacement will be necessary. Not the sort of thing I feel comfortable with myself - one of these days I'm going to find a beat up old handejector and learn how to strip everything and get it all back into place and working, but this is not the gun I want to practice on.