Also, how worn is the chain? A combination of worn chain + slight misalignment (+ possibly slight spec deviations) could be sufficient to give you issues.
On the bike I’m building now it’s a new chain and an old frame but it’s doing exactly the same thing as it was on the other bike (Shimano rear mech) which had an old chain an a new frame. Think I’ll take it to the LBS and see if they can get it shifting smooth because I’m done with it.
If anyone has an e*thirteen cassette they no longer need I’ll take it because it works perfectly on the Shimano bike.