It's probably rejecting the non-Campy shift cable.
You might try adjusting cable tension on the fourth cog (from the bottom or small cog). Read here:
http://www.bikeforums.net/showthread...ifting-problem
Lots of these problems have been addressed in the Mechanics Forum. Block out a few hours and get reading!
However, that being said, I rebuilt a Campy Mirage ergo (if I can do it, anyone can) and could never resolve a shifting issue. The only way I could get it to shift marginally well was to return to a stock cassette configuration (I had swapped out some cogs for lower gearing with aftermarket Miche cogs, but the tooth steps I chose obviously hadn't been approved by the Pope.)