nomis5 - 5:17 pm on Jul 22, 2012 (gmt 0)

One thing I believe for sure is don't make changes to your site in response to lower ranking SERPS position changes. It seems G understands when a ranking change occurs and connects this in the future to changes on the page. If your rank changes, stick it out for a month and then, if things are still the same, make a single change and see what happens. Don't be reactive.