You're describing what my son-in-law gets.
He gets real pale when it happens and passes out.
But his is because somthing in the brain tells his system that his blood pressure is high (which it isn't) and then sends a message to suddenly lower it. That's when he passes out.

I hope they find what is causing yours.
Not only is that frustrating, it can be dangerous if you are alone and fall against a hard, sharp object.

Let me introduce you to my multiple personalities. :)
     BluEcho...Faery_Light...Faery_Souls.