Have you taken a look to see if any of your unique sentences show results in Yahoo? It is possible that someone else is now using your content and you got dumped as duplicate. That's been discussed on this forum quite a bit under the name "scraper sites", although it gets misspelled with a second p many times. Some searches should get you a few threads on this topic.
The other thing to do is look for is this:
Yahoo searches for your specific pages that show someone else's. Oops... looks like ours got fixed. Anyway, it was showing someone else's domain because they had linked with a 302 redirect to our site. Business.com did this a while back and got our top rankings as organic listings of their redirect. Someone else did it just to that page and our page dropped out. 302 redirects are really bad for your site if someone else is using them.
If slurp is still slurping, then it shouldn't be a problem with the site. I'd try searching for some full URL's and sentences within Yahoo and see what that returns.
Brian.