I have some questions for you:
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How are you measuring search rank for page one? Not sure what the reality of that is.
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What does your Google Analytics say about actual traffic out of search?
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Do you pay attention to your Google Webmaster Tools for this site?
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When did you notice the problem and what were some of things you did prior to that?
In the meantime, I would get rid of email box as smartcow noted. This is such a horrific user experience (not being able to close it) that it must drive bounce rate way up, helping to tell Google that it is a bad search result.
After ditching the email box (maybe trigger it on a second page and allow for user to close it), fetch pages via GWT for Google to re-crawl. There is a pretty big difference between Google's cached result for your homepage and the homepage as it is.
Without actual evidence of some black hat plot to kill your site, I suggest keeping an open mind to less nefarious/more likely possibilities. Fear can decrease your perception of facts.
I look forward to your answers to the above.
Best of Luck!