Michael_T - 6:18 pm on Feb 1, 2013 (gmt 0)
Check to see if any of your bigger changes in traffic coincided with Google's Panda or Penguin updates. If so that would give clues.
This site keeps track of the dates for Google Panda / Penguin
For example, we had a huge hit around March 24, 2012. Our main key words dropped from #2 position to the 3rd page.
That date coincided with a big Google change that I think had to do with over optimization and too many links to you with the same link text.
Also look to see which of your keywords dropped in the search engine rankings. In my case they seemed to have penalized me on specific key words, which also happen to be the keywords in many of the links pointing to us.