How frustrating that there's no clear sign of what's causing them. My mom's were always unknown (not that she ever really went to the doctor to find out), but we figured out after many years that hot dogs caused my brother's. Sounds weird, but something about the hot dogs (no matter the brand) that gave him terrible migraines. Do you eat hot dogs? Just kidding.

Hopefully you'll be able to figure out the cause so that you can not be exposed to, or fix the things that cause them. I know the brain is one tricky puzzle, but that's what neurologists get the big bucks for.
Take care of yourself!