I had this problem when replacing the stock rings on a crank for some that are custom-sized. They were spaced / offset slightly differently than the originals, and so the chain could often skate between the middle and big rings without being engaged. This couldn't happen with the stock rings because there was not as much space between them. Since you said that the rings had been changed, I would first look to see if they are the problem - even if they are also Campy rings then they could have been designed for a different crank since they were bought 6 years later. So, try some different rings and see if this fixes it.