My sisters does have PKU, and she has been on the diet since four days of age. The tremors developed when she was only a few months old. They were not typical tremors that involve the hands being shaky or unsteady. Instead they were tremors in which several times a day her leg would shake uncontrollably for a few seconds, and then it would stop. The neurologist was never able to figure out what was wrong, and the tremors went away on their own within a few months. We were never able to determine whether or not they were related to PKU.
We have decided to not worry about it anymore, because today she is one of the brightest toddlers I have ever met LOL, and she has been developing like she should be. However it still would have been nice if at the time the new doctor who was following our care would have helped us out.