Sounds familiar. My first experience was when I was maybe seven years
old - that time I stood dumfounded until I actually did pass out.
Since then I've never completely lost consciousness, though some times
that requires lowering my head to increase blood pressure to the
brain.

My experience is that my vision pixellates away to a plain dark gray,
starting in the far periphery and slowly closing in toward central
vision. Sort of like the shrinking "HBO" logo at the end of many of
their programs...   I've learned to have a bit of control over how
quickly it happens and how far it progresses, though telling someone
how to do that would be about as easy as telling them how to control
an orgasm.

I've had these experiences for fifty years now, and can't tell that
they've hurt anything. No MD has ever admitted to recognizing them.
The only recognition I've found was from psychics, who say it means
you are "leaving your body". You might want to explore the "OOBE"
literature, since you seem to have verified that the MDs don't see a
physical problem.

Loren