Thanks guys, I did check the spacing and indexing with extreme care. It had been working just fine in the weeks prior to the start of this issue, perfectly in fact. I am extremely good at dialing in the indexing, and the cassette spacers are right. Good point about the cable attachment, I know what you mean about Sheldon's "alternate" cable routing. I had been using Powercordz, which have to be wrapped all the way around the anchor bolt, but still the actual attachment point was the correct one (and had been functioning fine in the weeks prior). To make doubly sure, I replaced the Powercordz cable with a die-drawn slick stainless cable, and the problem is exactly the same.