Why am I getting awful headaches?

[Twyla]

My head first starts off with a pounding, located generally in the back or in one of my temples. It throbs for an awful amount of time and makes me shake and they become so horrible that I have no choice sometimes but to go to the hospital just to get some relief. No medicine I have taken (Advil, Aleve, Excedrin, Tylenol) has done me any good; it can't be my diet or my sleeping habits because no matter what "long term" fix I have tried, nothing has worked. I tried to decrease my sodium intake or my caffiene intake but that didn't work either. Yet every doctor has told me it has something to do with my diet or sleeping habits. I don't understand, because every MRI and Cat Scan I have had has brought up nothing. How can I end the pain and what is it from? Please, someone help. =(