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It might be worn out chainrings! Many rings are run far beyond when they are “worn out”. Seriously warn rings will show deep cupping in the areas between the teeth. Often a chain that is long worn out will have grooved itself into the chainring in such a way that it all still works (mostly), but if the chain is replaced, the new one will not function well. This is a bad situation because it indicates replacement level wear on the chainrings, but also probably means the rear cassette needs to be replaced.
