Yea, Adsense eCPM always changes on me so the best advice I would give you is not to rely on one network and rely on more. Possibly if you add another network that gives you a CPM of $1-$2, then that would back up all the loses you had with Adsense dropping. Just a tip.
I think, as hinted upon in the first post, more and more users are becoming accustomed to the whole Adense system. Also, Adsense is the in the affiliate business now. Could be an issue now, but I think it is more of a future thing. Lastly, Google's clamp down on shady ad placement is probably leading to the decline in Adsense revenue as well.