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Bad Lag, please listen to experience. I've had this happen dozens of times because I was a professional bike mechanic for several years. Effectively, I have the experience of thousands of cyclists. Others here have seen this, too. We know the cause, and we know the solution, and I think you would do well to pay attention. This thread has so many complicated theories and esoteric procedures that I don't think help. You often have to change the chain and freewheel together. You may not have needed to until now, but now you do. Accept that, change the freewheel, and learn a lesson. It's not that complicated. The wear may not be visible, but it's there.