incrediBILL - 4:00 am on Oct 29, 2011 (gmt 0)
It's the bidding system and advertisers causing what perceives to be throttling.
Google puts up the best paying stuff first, when those budgets runs out you get the dregs, so it appears to be throttling and it is, the advertisers throttle their budgets.
The next best bidders budget runs out, so on and so forth until you get the bottom of the barrel and it repeats day after day.
Some sites probably get the lions share of the better budgets too as the algo determines that site provides the best ROI for the advertiser.
I've said it before and I'll say it again, if you want the most AdSense money, you have to build a site that incites more buyers to convert to sales.