Are your slideshows at the top of the page? Read this tweet (from the czar himself) which says there was a refresh Feb. 6th of the page layout algorithm. Cutts: https://twitter.com/mattcutts/status/432940645200588800

It targets ads above the fold, but seems to indicate that lack of written content within easy viewing reach can also be undesirable.

Read more about the algorithm here: http://insidesearch.blogspot.com/2012/01/page-layout-algorithm-improvement.html

Other possibilities may be that the text in your slideshows isn't html, or you don't have alt tags for the images, or the slideshow slows down the site, or that's all you really have for content on the site? Some slideshows are just not SEO-friendly, aside from algorithms and the case that they are generally not user-directed, and hence not user-friendly.

Good luck! Sounds like the the kind of haystack I find myself rummaging through as we speak;)

Wow, you are very quickly and easily making me much better at what I do:) Thanks for that.

I actually just updated the code a couple days ago by adding the Expires code and fixing the redirect. Maybe the previous double 301 redirect could be the culprit? Or - something I mentioned in another question - there were a ton of 404s because of a blog that wasn't redirected to the /blog subdirectory correctly, which I fixed recently. Could something like that cause the server to work to hard and return a 500 server error?

I'll definitely check out the logs and Pingdom.

Great information and advice.