Should replacing the rear derailleur on my 80s road bike with a Campy Nuovo improve shifting?
Take the shifter cable out and lubricate it, or replace it if it looks like it needs to be replaced. If the pivots are as well maintained as you say they are, it could be the cassette that needs to be replaced. Most of the time you change an old chain, you need to change the cassette for things to continue to be smooth.

From what you describe, the SX410 is super old, it might be worth it to have a mechanic take a look at it to make sure its properly greased. Once you've properly ruled out those problems, then try the Campy.