Low Back Pain and Tremors

My father is 57 years old and suffers from chronic lower back pain and tremors. The more pain he is in, the more he will shake. He has had the tremors for about 3 years. His back pain is getting worse, so his tremors are out of control. He has tremors in his trunk, but the past month they have moved to his right hand and leg. He has a morphine pain pump and neurostimulator implanted. He also takes valium 3 times per day. We have been to about 10 different neurologists and neurosurgeons. Now we are only seeing a pain mgmt doctor. Anyone ever heard of these symptoms?