Every Few Months I Get These Spasms Where I Am

Every few months I get these spasms where I am completely awake and aware, but my body just shakes uncontrollably. I completely lose my balance while it happens and it lasts for about 20 seconds. I originally thought it could be Due to lack of sleep and not eating right, but the past couple times I'd been resting and eating well. It's been happening for going on 4 years now. Doesn't get worse, but doesn't get better. What should I do? What kind of doctor should I contact?