In weekends (Sat/Sun) one of my website goes up in search results by about 4 positions and comes down by 4 positions in weekdays. In weekdays it is on 8th position for a keyword and in weekend it is on 4th position. I've been observing this for past 6 months or so. What could be the reason? Does it hint that Google is deliberately trying to modify the search results? At least that is what I can conclude from my observation.
Perhaps it is what they do to test the credibility of other results? After all, there is no use in just showing the same results, regardless of backlinks. I'm sure it's perfectly normal. Mine fluctuates as well.