I think I might have a rather specific problem. None of my searches came up with anything. So I'm working on a Shimano 3-Speed Nexus hub. The owner bought the bike new a little less than a year ago. It is shifting fine. However, if she keeps it in 2nd for a while it will start making noise like the chain is about to fall off. Sometimes it slips down to 1st and then pops back up. She has taken it to the shop where she bought it a few times, but they have just made it worse. Any ideas?