One cause of seizures in frogs is a calcium deficiency. You should dust your food items with a good Ca supplement before feeding. Other possibilities could be parasites or liver damage. A vet could analyze a fecal sample to determine if the frog is infected. Liver damage may be the result of overfeeding. In any case, you should seek an advice of a vet. If a vet isn't nearby, check with the biology department at a nearby college or high school and see if they can analyze the sample.