Why is my chain skipping?
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.