I been running adsense for many months and when I start looking at the earnings, click rates, and number of clicks I can't help but sense a pattern. It seems that regardless of the number of clicks, my earnings are pretty much the same. ie double the clicks don't produce double the earnings. Something also kinda interesting I noticed by running some click tracking software, is that my clicks tend to come in groups, and not from the same user. In one 20 minute period I might get X number of clicks from Y number of unique users, then dead space for hours and then another grouping. I should mention that my site has steady traffic the entire day. Is adsense doing something to throttle the ads and therefore the earnings?
Is it possible that this is because you are getting particularly interesting/relevant ads for 20 minutes or so and then the ads change to something less interesting/relevant for a while? If that's the case maybe you can figure out what the ads are that attract the clicks and try to see if you can play with your content a bit to make them show up more often...or block their competitors to get them more time on your page.