I feel for you. We have a cat that looks just like yours have some very bad seizures. We could not track down what caused them and blood work came back fine. The vet gave us some natural medicine that helped and he hasn't had much of anything serious since. Just a few spurts of hyperness and weird twitching.
I am not sure what the name of the medicine was or if it would work for dogs. Although, I will ask my better half and get back to you.