Thanks. I had a torturous day with this thing. I listened to what you said about chain-line, and tried putting a spacer behind the freewheel, trying another (just-as-used) freewheel and moving the front chainring outboard a bit. I tried bending the body of the derailleur to get it more precise and I tested every chain link for a frozen one. It's probably something simple like a worn chain or worn sprocket, or both. Thanks for attempting to answer my question, though. It gave me something to think about. It seems to me that the significant question is: Why just the low sprocket? I looked closely at the chain and it seemed that when I got some chattering, it was when the chain looked like it wanted to jump off the side of the lower (guide) pulley. I made an appointment at my local co-op, who are the only ones who know how to work on older bikes, but that is not till May 12. I think I'm going to try my luck with putting on my Benelux Mk7 after I calm down and relax a bit. I can show you a video if you think that would help.