It works now.

I took the derailleur off and cleaned all the gunk out of every nook and cranny of it and the pulleys. But that didn't help. So today I went and got a full Campy cable and housing set. In white. It looks hot.

And now it works fine.

Sigh...Campy...is there anything it can't do? So I'm guessing that the year old housing had gunk in it, affecting the travel of the cable. Or maybe the cable itself had some sort of failure. Probably something on your list was it, Bob.

A guy at a bike shop suggested that it sounded like a bent derailleur hanger, too. I'd actually be interested in getting that checked, as the bottom pulley does look like it's off-centre of the chain by a little bit.