Thanks for the responses. I have not seen an electrophysiologist. In fact, I've never even heard of that specialty. So thanks for the tip. I'll look into this a bit more. I've had so many tests done and have been searching for a diagnosis for this thing for over 15 years. If anyone has any ideas as to what else I should look into I would be most grateful. I wouldn't consider this medical advice, just areas to investigate. I've been searching for an answer for too long to panic over possible hypothetical causes, so please don't worry about causing me any anxiety with your suggestions. My symptoms are brain fog and mental fatigue, especially after exercise. But I usually do fine while I'm exercising. Overall, I seem to do better on a high carb diet, and a low carb diet leaves me feeling run down. I've looked into hypoglycemia, mitochondrial disease, and all of the other standard stuff and all of my tests have come back normal. Thanks again for any and all suggestions.