tedster - 10:46 pm on Dec 17, 2010 (gmt 0)

1500 1600 1700 1800 1900 2000 1500 1600 1700 1800 1900 2000 1500 etc Each number represents about one week.

So that's not tilted toward the weekend days, its tilted toward every sixth week? That is peculiar indeed. Some kind of statistical testing and traffic throttling does come to mind, but I'd dig into the stats pretty deeply before deciding on that explanation.

Are you seeing this behavior on any particular query term?

If I get extra traffic from other sources (then Google) it seems to be compensated.

So the numbers you are reporting here are for ALL traffic, not just Google search traffic?