What Causes Tremors, Loss Of Balance And Nausea?

My father is 72 and, until the last 6 months, has been relatively healthy and active- DESPITE the fact that he is a smoker. We live in Boone, NC and my parents migrate south, to Florida, right after New Years. This past year, before they went to Florida, dad complained about a light film/wetness in his ears every morning. Then, he started feeling nauseous and has pretty much been that way since January. He has been to Dr. after Dr.- ENT, Primary Care, Gastroenterologist, and Heart Specialist. He has had an MRI, endoscopy, colonoscopy and blood work and the only thing he ended up being treated for was depression (over moving out of and selling their house of 35 years) and and h phylori. In the last two months, he has started shaking and just this week has begun losing his balance. THERE HAS GOT TO BE SOMETHING THEY AREN T FINDING! Do you have any idea of what it could be or what kind of Dr. we need to take him to??