I noticed that you said you stopped all vitamin supplements to see if they were causing a problem. You might want to consider putting back: CoQ10, B50, Calcium and Magnesium. All of these are said to help with migraine. IMHO, it would be doubtful that these things would be problematic and they also would have other significant benefits.
Other supplements that are good possibilities include: Butterbur and Feverfew.
IMHO, it often takes a multi discipline a approach to make headway into this often difficult problem.