I assume you mean it does index, but doesn't trim correctly. Otherwise please explain better with more specific description of exactly what it's doing.

I suspect the issue may be friction in the housings. When you downshift vis the paddle, the RD overshifts slightly, then is supposed to settle into trim (all Campag downshifting (to larger end of cassette) is based on overshift and return to trim). Cable friction, and sometimes gummy action within the lever, causes it to stick in the overshifted position so the trim is off.

My favorite diagnostic for this is to shift, then pluck the bare wire away from the frame like a guitar string. If that corrects trim, you know it's a simple friction issue. Field strip the cables and clean and lube, ot more likely given the age, replace the cables and housings with new.