Many publishers have found large fluctuations in their AdSense revenue and have wondered what is causing it and if there was anything that could be done about it.
Google has put some information together in two parts that they say should be helpful.
This is Part 1
Inside AdSense: Diagnosing and treating revenue fluctuations (Part I)
This is Part 2
Inside AdSense: Diagnosing and treating revenue fluctuations (Part II)