"The only thing I've done differently with the 2 keywords that are still doing well, was more focus - we did more link-building for those, but not an extreme amount. Never over-optimize. This is why I really don't "think" it's a back-links thing..."
How optimised are the pages? We are finding that if you have the keyword in the URL, the title, the meta description, the main heading, the subheadings, etc (i.e. every part of the page) then it is likely that last week saw you drop for that keyword.