econman - 1:27 pm on Mar 6, 2011 (gmt 0)
positions are constantly changing every couple of days
I've seen this pattern in the past. It looked to me like Google might be doing some sort of elaborate multivariate testing -- generating and analyzing sample data which allows them to run all sorts of statistical tests, comparing CTRs, frequency of users returning to try the same search again, etc.
I've even seen this pattern of position swapping on an intraday basis -- our page usually appears in position 9 but I'll see it appear in position 5 for a few minutes at random points during the day, (or at least that's the impression I'm getting -- but I haven't been curious enough to actually check 300 times at 5 minute intervals.)