Chain tension or slack was my first thought. I'd tighten the cage pivot spring a little if possible on this derailleur. You went to the B-screw. Does the same thing but also changes the relationship between the derailleur and the cassette which may or may not be for the better.

The old SunTour derailleurs had the outboard (in relation to the bike; deep in the pivot housing) pivot spring sitting in one of several holes. To tighten, you just lifted the spring (after taking off the cage) and rotated it slightly to a tighter setting. While I was in there, I'd also clean and lube. This was about a once every 10 years operation. FB's right about the pulley but I'm not sure I've ever seen one get to the point of shedding chains.