I thought it might be good to note here that there is a growing feeling that constant small tweaks to the on-page content might be causing ranking drops. Some obervations from our February 2008 Google SERP Changes [webmasterworld.com] thread:
Tedster: Working with this idea a bit, I've noticed that it may NOT be a good idea to try to rank one page for all those "related searches" at the bottom of the SERP. I know of several attempts that were followed by a ranking drop a few days later.
Robert_Charlton: I'm wondering whether Google is suddenly more negatively sensitive than it used to be about onpage changes that involve modifiers to targeted phrases... even if the change isn't actually changing the targeted phrase, but involves changing an adjective that's adjacent to it.
potentialgeeek: One of the main ways Google scrutinizes "SEO sites" (sites optimizing for competitive targets) is how often they change, and what type of changes are made, I suspect. Are the changes natural? Many on-page adjustments aren't reasonably justified unless they're only for SEO.
I'm seeing sites that fiddle a lot are getting kicked.