Originally Posted by
dsaul
2000 or 2001 makes a big difference as the 9 speed system changed in 2001 and is not compatible with the previous 1997-2000 9 speed system. I suspect that the derailleur and shifter are not compatible.
That's not relevant to the problem of staying in gear. The difference is entirely in the index cam with the other parts staying the same. Appropriate vintage spare parts manuals list the officially required substitutions.
The change was in the actuation ratio from 1.5 (8/9 speed) to 1.4 (10 speed, which needs to move farther to get to the big cog which hangs off the end although the cable drum diameter is limited by the shifter body dimensions so the ratio must change) to reduce the number of derailleur bodies Campagnolo had to make. While not officially compatible for 9 cogs (just for 8 cogs provided that a narrower 9 speed chain is used to match the pulley width) many people find a new ratio derailleur works well enough with an old ratio index cam. Old derailleur with a new shifter would be less problematic because it'd produce an over-shift not an under-shift.