It might well be a good time to check the cable where it joins the rear der. I don't know how important it is generally, but my rear der fitting instructions said to run the cable over the top of the bolt and not underneath it. Maybe that is useful to know, maybe not.

The cable might need to be pulled tightish over the bolt before it is locked into place. Some folk use pliars to give it a good pull before the bolt is tightened. I tried that and my shifting was a little unpredictable. Eventually, I settled on pulling the cable by hand and getting enough tension on it to be useful but not so much that the cable is as taught as highly strung wire. That seemed to help a lot and all other problems in shifting were then corrected on the adjuster.

Is the cable routed ok around the frame? That is, there are no 'pinch' spots as it were. I recall Sheldon Brown documented the correct cable routing. Does the cable fit securely onto the frame using the lugs it might have on yours?