Explanation from Google

The support folks at Google sent me a very nice message today answering my questions about why my web site seemed to periodically disappear from their index.

We update our index every four weeks. Each time we update our index we find new sites, lose some sites and sites shift in their ranking. If you happen to enter the same query while we are in the process of posting the index at our various data centers around the country it might seem like you are seeing inconsistent results from Google. What is actually happening is that you are seeing a result from an “old” version of our index one time and a result from a “new” version the next. Due to the size of our index, we can not simultaneously post a new index at all of our data centers, which may result in the behavior for a short period of time.

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