Well, you know the answer. More testing is needed to make a diagnosis. I would go to different vet in your area and get another opinion. Better yet consider consulting an internal medicine specialist or a neurologist, whichever the vet that examined him thinks would be best.
Also if you can video the symptoms when they occur on a camera phone or something to show the vet it would be helpful.
I would stop focusing on the food as it may have nothing to do with causing these attacks.
Also, be careful about taking advice that you get on the internet and forums. You will have better luck listening to a vet that has actually examined the dog and knows his history.
As you can see from the links I provided, people sometimes go around and around and often get nowhere. It can really add to the confusion.
