One of my sites has had a ecpm drop- I know people attribute such drops to the BigDaddy update and so on.. But I also have my own conspiracy theory :-) This site of mine hosted a phpBB forum that got hacked and traffic was diverted to some junk sites. It took over a couple of weeks for me to notice and correct this..Do you think bad traffic generated by the hacked pages resulted in the lower ecpm for adsense (smartpricing etc)?
I assume that you are getting some targetted ads appearing on your pages (even if poorly targetted)?
If so, I'd suggest that, given time, your site will recover from any spoiling effect that these links have had on your site. It can take a few weeks for the Adsense robot to get the best serving ads for you.
Alternatively, it might just be the natural ebb and flow of eCPM. In which case, just leave things as they are and you'll get better results at some point.
If you make unnecessary changes, it might interfere with the Adsense robot's task of maximising your site income and suppress your income for even longer.