Hi there,
I've been with Adsense since 2004. At 2005, I got some very good earnings and they stayed steady until 2012. Some culprits were Panda/Penguin, and then later some changes to the algos, they began tweaking the detection of missclicks. At the end of 2012, my site went to 50% revenue just from November to December. At 2014, I was at 30%. At December 2014, 10%. And now, January, I am at 4%.
I am not sure if my domain is being punished somehow, or if it is my account (my father has a small site with similar smaller content, and it has similar earnings now... I have tons of content, and he has one section with few visitors).
I don't know if I should throw the towel. Or if I should start a new website, or rework the whole site. :S