Sometimes I think the bike-gods just laugh at us. I am have all kinds of problems with a new Dura Ace chain/cassette. Like you never had a problem with Ultegra. Grrrr.

So are you actually dumping the chain into the spokes?

It sounds so obvious, but have the LBS check the limit screw on the rear der. again.

If that doesn't help, perhaps it's time for some sort of "bike voodoo". Burn photos of Mario Cipollini whilst covered in Phil Wood grease and chanting the Campy gruppos at the next full moon or something. Can't hurt.