Brands of housing/cable make a difference. I had changed to a Bontrager set and the shifting was awful. Went back to generic and the shifting got much better.
Could also be the rear der is worn. Pivots eventually wear making for sloppy movement. Check by twisting the der and checking for play.
By the way, when I had the shifting probles, I checked the chain and it was near the wear limit so I changed it. Made no difference in shifting, but it can't hurt to change it.
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Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1