Originally Posted by
Davet
It's not skipping on the upper pulley, only the lower, which makes suspect the lower pulley is binding and not turning freely.
Or the pulley teeth are well worn. Or the hanger/cage is really bent, or the chain has stiff links/lube, or some massive cross chaining angle is at play, or... But I will say that the OP hasn't yet really assessed the state of the system in a way that many possibilities can be eliminated. Like many problem one must go through the various possibilities to filter out the static. Andy