The thing it sounds MOST like is mini-strokes, sometimes called transient ischemic attacks. They don't show up on an mri, because they are like tiny strokes that didn't totally happen. It's more a diagnosis from examination and history. Neurological deficits like this that come on suddenly and then go away suddenly are scarey, and something scarey might be happening, even tough it's not yet diagnosed.

I'm hoping someone listed to his carotid arteries to see if they have plaque, and is investigating this deeply.