I have had a positional-dependent tremor in my left hand since I was a teenager in college. It only occurs when my elbow is at my side and when my hand is approximately level with my navel and about three inches away from my body.
When I position my hand in the "sweet spot" (and it often takes me a number of tries to hit it just right), it trembles like I have Parkinson's (which killed my maternal grandmother, or so I am told). But, if the hand is a half-inch out of position in any direction, my hand is as steady as a rock.
It was evaluated by neurologists back when I was in the Army, as well as during a fitness for duty physical with the civilian government back in the 1970's. Both came up with nothing (the first suspected me of malingering, the second of wasting their time).
And, I have one index finger that is noticeably shorter than the other - this actually affects my right hand positioning on most clarinets.