I can see that this thread is a few months old now (I'm new here so just browsing around), however I've had similar experiences with a friend of mine - although not whilst diving.
She was fit and healthy (a dancer), around 20 years old and from time to time she simply just passed out, was unconcious for a few minutes, still breathing, was a bit confused and maybe a little tried as she came to - then she was fine again. There were never any warning signals. She never felt sick before or more tired on the day, dizzy etc.
This just happened one day, a while before I met her, and she was going through a lot of medical tests to figure out what this was. It then happened at irregular intervals, and several times throughout the 10 months we were in school together. The doctors seemed to think it was a form of epilepsy, and the fainting was a form of a seizure. But at the time it was just guesswork - all tests showed she was perfectly healthy, although something was definitely wrong. They didn't have any cure or way to treat her when I last spoke to her a few years back - maybe they do now?
Just wanted to let you guys know - it was a mystery to the doctors here, and if they've figured it all out now it still would have taken years for them to find out what was wrong.