Check the Campy NR RD pulleys for cracks. They tend to do this with age.
Check the BB and chainstay areas for cracks, since the bike is older.

Lube the friction shifter, cable housings, inspect the shift cable or replace it with a newer die-drawn one.

There must a lot of flex, if it is BB flex, to ghost shift a 5 or 6 speed freewheel.