Sounds like insufficient tension of the shift lever wingnut. Campy shifter levers were really primitive and did a terrible job of holding adjustment so that there was enough friction to keep the RD from slipping or having too much friction to shift lightly. Much better use for their shift levers was as a keychain ornament. Common and acceptable fix was to replace the campy shifter levers with spring/ratchet simplex or suntour shifter levers that actually work. Short of that, try tightening them down more but expect them to feel stiff and randomly loose tension adjustment.