one of my sites (100%white hat, with only whitehat backlinks, 4 years old with over 600 unique pages) has been dancing like hell over the last 8 days. one day the traffic from google is 3500 uvs per day, the next day it falls to 67 uvs, and again there is a raise in ranking. Every other day it happened.
And like someone above said, there are some empty sites (don't even have a homepage) are ranking in the first page at times. This is ridiculous, but I have moved on and not relying on google any more. If traffic comes from google it is just a bonus for me.