I'm gonna throw up this video- didn't even really watch it, but it looks good
Should talk about what the b screw does and does not do.
Still want to see the chain tension. I've never had a chain stretch so much the derailleur can't keep it tight- not in 3000 miles. It might have been on the long end to start with.
Weird.
Dang, wish I was there. This is the kind of thing, if Look or one of us was there, it'd be fixed easy peasy. It's tough sometimes diagnosing over the interweb.