Throbbing pain in gold crowned molar

I have had a lot of problems with this one molar.  I had a root canal done several years ago, and then a gold crown.  Within a year of that procedure, the pain would come and go --- was seen by an endodentist who drilled through the gold crown and did another root canal on the same tooth, saying the dentist didn't go deep enough into the root.  It has NEVER really subsided.  I'm at the point where I just want this pain to stop.  It hurts by itself, without chewing, etc.  Also, it has ALWAYS been sensitive when touched or tapped.  My dentist constantly tells me there's nothing there to be seen --- no decay --- nothing he can do.  HELP!!!