Should replacing the rear derailleur on my 80s road bike with a Campy Nuovo improve shifting? I'm considering replacing the Simplex SX410 derailleur for a Campy Nuovo Record on my early 80s Mercier (6-speed cassette).  Reason being that there has been for several years now a problem shifting from the 3rd to the 4th (and vise versa) one needs to move the friction shifter more than for the other changes.  Can I expect more precise shifting with the Campy?

I have recently changed the chain too. I have a SRAM pc830 now.
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.