I strongly believe traffic throttling is occurring by G putting a ceiling on how many impressions per keyword a website receives. Our WMT impressions graph is simply far too consistent to be believed, and seems to reflect some sort of testing.
If I may explain testing. Our WMT impressions graph will show six days exactly at 10k impressions, then one day at 8K impressions, it will repeat that pattern three times, then we see six days at 8K impressions, on day at 6K impression repeated three times and then...
Sorry but there is no statistical way in the world that it can be anything but traffic throttling by throttling impressions. More than happy to email anyone the snip from WMT showing this.