potentialgeek - 8:46 am on Mar 5, 2007 (gmt 0)

* A high-end AdWords Ad Campaign ends. You'll see advertisers throw money at online advertising from their ad budget. They don't necessarily spread it thin, because that's not the most aggressive advertising. They bid high to get top spot. So you see weeks or even months with their ad showing up at the top/left side of the ad units. Then their exhaust their budget, and, poof!

* Google "Quality Skewers" the advertisers and they're off to YPN! :/

p/g

P.S. I suspect SERPs make a difference. If your competitors ahead of you have Adsense ads, and they are similar ad units or similarly placed on web pages, or with similar blending/colors, you won't get as many clicks. (They already clicked on the same ads, been there, done that.)