Gab Goldenberg from SEO ROI posted a thread at Google Webmasters Help discussion forums when he noticed Google began to drop his rankings for all his pages. I have been tracking the thread for about three weeks, but held off on posting about it until Gabs wrote his post.
How I Cloaked My Way To LOWER Rankings was Gab's title and it shows how you must be careful about who you request site changes from. One WordPress plugin conflicted with a custom plugin and it turned out to generate cloaked-like pages in the eyes of Google. Google would see virtually the same title, content, etc for all the pages on the site, while a human would see unique pages.
Clearly, this type of duplicate content issue, with cloaking, is a recipe for disaster in the search engines.
Thanks to the good help of the folks at Google Webmasters Help and for Googler JohnMu pointing out the issue, Gab is able to address the issue and fix his site, which will fix his rankings.
Lesson learned: Test your changes in a test environment and try your best to understand how the changes will impact your pages. Some times things slip through, like in this case, but don't be afraid to ask for help.
Forum discussion at Google Webmasters Help.