If that's it, you have to learn the triggers, and when
you feel it coming on lay flat. Blood
flows faster to your brain when
you are flat. It's actually in improper response - instead of your heart rate pumping faster when you are getting up and down (doing things and get hurt, etc) for example, it actually drops off/low, and literally drops you. It's your body defense mechanism to get that blood where it needs to be.
However, they can give you a tilt table test for that to know for sure.