Good question, and it is one we have considered pretty carefully.
The initial drop was closer to the Panda update but at least a few weeks or even a month or so after it hit all the content mills really hard—it also went along with the huge shift in our keywords and all the risqué content in our forum. Blocking that content caused us to recover our search engine positioning which seems inconsistent with Panda. And today our second drop takes place again despite almost no changes made to the main site.
I read the 'charter' of this forum and I recall reading that we're not to be too specific about our website, which makes it a little difficult to troubleshoot this. Perhaps this can give a good picture without breaking any rules. We are a commercial sales company functioning within a specific but smaller industry. Our site is broken down into sections for rentals, sales, service and support. The only aspect of Panda which concerned me to some extent is the fact that many of the models in our sales section use some content which comes from promotional material (content which various competitors also use) so there will be some degree of duplicate content in this regard across the internet. I'm attempting to address to some extent with a big upgrade I'm working on (it is difficult when you're dealing with descriptions of some pretty technical equipment) but if this is having any impact, it is probably an opportunity in disguise. I don't think it is Panda, though, because relative to the rest of the website this content is not great, many detailed top-ranking pages with unique content are suffering alongside everything else, and the dips/drops don't coincide very well with anything I've read about Panda.