I have seen these fluctuations too. It seems that new domains fluctuate more than old domains. Also Mike maybe halomega never ranked well previously and now since you are optimizing it, G is looking at it as new. Jaan, I notice that webmaster tools mis-reports ranking too it is almost useless when you can't trust it. It's google for christs sake, you would think they would report their own data accurately. Logically, it must be a datacenter issue. It is almost like google is trying a "quality score" type of thing with natural rankings in different datacenters and updating. I noticed that when I launched a new site with no ads of any sort, i would first start to see traffic from google organic coming from a specific region of the US. Another thought..maybe you are not considering is the adjustments and fluctuations of the competition and google's reaction to those adjustments. - my rant/2 cents.