I see no problems in the source code, such as HTML tags inside the head section. All in one SEO seems to be working. W3C throws out a few errors but nothing too serious as far as I can see, most of them caused by a "link enhancer"
http://validator.w3.org/check?uri=ht...Inline&group=0
This should not affect search engine rankings. I did however notice that some of your pages are listed in Google in an odd way:
http://www.google.com/search?q=site:...n.com&start=10
The single post page "Descent into Chaos", 5th result from the top, is listed with the URL
relaxandhavefun.com/?p=43
=1
although it should be
relaxandhavefun.com/?p=43
http://www.google.com/search?q=site:...un.com&start=0
The single post page "Orientalism|Book Reviews", 2nd result from the top, is listed with the URL
relaxandhavefun.com/?p=27
%22
although it should be
berelaxandhavefun.com/?p=27
%22 is the Hex code for a double quote. Looks like that specific post once was linked as href="link"" (with 2 double quotes at the end)
There may be more like this. I have not looked further than page 5 in Google.
Atahualpa does not seem to generate those =1 and %22 appendices. I could not see those pages linked from your site that way.
While these URL issues indicate that something is wrong (or rather: was wrong) it may not be the (sole) reason for the traffic drop you're seeing, this can have 100 other reasons.