Hi Keri,
You're right, those graphs are for the entire site and not just Google Organic however it's the same (worse) when you just look at the organic traffic. They don't do any form of other marketing so it's not that sadly (I truly wish it was!).
Here's an overview of just the Google Organic traffic: http://www.thesitedoctor.co.uk/img/201202080932-Google-Analytics.png
Although there is a similar drop in traffic from Bing/Yahoo, there's nothing so cyclical as this.
Nothing in Webmaster Tools other than the usual notices about pages blocked by robots.txt.
We'll dig some more into the individual pages to see if there's anything in there thanks that's a good idea.
Tim