Participants will be randomly assigned to receive infusions of either IVIG or placebo (an inactive substance) through an arm vein once a month for two months. The infusions will be given in the hospital in doses divided over 2 days, each lasting 6 to 10 hours. Before the infusions, patients will undergo ataxia assessments through tests of coordination and balance that may involve finger tapping, walking in a straight line, talking, and eye movements. When the treatment is finished, patients will be followed in the clinic once a month for 3 months for blood tests repeat ataxia assessments to evaluate the effects of treatment.