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A few days ago it seemed my site restructuring efforts had paid off when I came second for a particular phrase (out of about 5000). But after sales started dropping off recently I decided to check my phrase again: I went from having my two main pages at #2 and #3 to having a far less important page at #11 with no sign of the others! This was quite annoying as I couldn't imagine what I had done wrong, but then I checked the cache of the page at #11 and it seems that it has reverted to the really old version?!
I don't see why google would update its cache and then drop it for an older version - it has nothing to gain by not being up to date surely. Zis iz craaaaazzeeee you know?
No worries, it's a pretty normal thing for google to do. They usually flick between new and older pages and with every "update" the newer version becomes the older version if you know what i mean ;)
I highly recommend out the google forum knowledge base [webmasterworld.com] at the top of the board, loads of heads up and tips about the inner workings of google
When PR is recalculated at every update, you can be more or less assured the results are solid, and will look like that for the rest of the month, including cache.
Things change here and there but most of the shuffling is done at the "update" period.