swa66 - 12:15 am on Apr 24, 2013 (gmt 0)
"GooScrood", "a bot that's clicking on ads", ... to put it to rest with one simple concept: Occam's razor.
So what's the simple explanation: something changed, either an advertiser (e.g. one targeting your site) dropped out, either something in the looks of the ads changed (e.g. removal of the ugly arrows) that made them less attractive to click on for your visitors.
Now even before this drop, you already had a lowish CTR IMHO ... SO I'd invest a bit in A/B testing to see if you can;t change the layout of your pages to increase this. Some ideas can be found
and the old heatmap: