Could you comment in the other thread [webmasterworld.com] where I speculate how the two different cache's might function? I can't tell whether your experience is the same as mine, but I can repeatedly demonstrate the presence of two caches. (PM me if you are interested in the actual case which shows the cache is selected deterministically, not randomly.)
As you try to figure out the weirdness, take a look at the cache'd page which is being used at that particular time. My guess is that your site's position is dependent on the cache contents, and that the cache used is dependent on multiple factors, including search phrase. And I sure hope that minor page tweaks won't shift position by dozens of places.