What Causes Chronic Headaches At The Back Of The Head?

I am a 42 year old male and have been getting terrible, debilitating headaches for the last ten years. I had an MRI done in 2005 and a CT scan in 2007. No abnormalities were found. I have taken several meds including Zomig, Naproxen, Relpac plus many others. Nothing seems to work for me. Should I get another MRI or CT scan done now? Most of these headaches start in the back of the head. I am not sure where else to turn. Been to the ER three times for this, but I still suffer. Do you have any advice or thoughts? Thanks Tim