Just a guess but the problem might be solved by switching the cables from Shimano to Campy or some other 1.1mm cables. The coated Shimano cables are close to 1.2mm and the housings are close to 1.2mm.

I had a similar problem with new cable and housing on my Dura Ace equipped 91Klein. I thought it was the 7400 derailleur but once I ditched the Shimano cables it shifts like butter.