Mine feels like very bad orthostatic hypotension, except it comes on out of nowhere without a position change. On days when this happens I usually feel "off" the whole day long, and when I feel like that I am extra careful to rest and if I start to get that feeling I sit down right away. I've gotten better at managing it so that it doesn't happen badly anymore, I am able to cut it off as soon as it starts.
I also have a dizziness that comes with flares, and I have a "brain fog" and trouble remembering things as well. The dizziness feels like I am on a boat, or just got off a boat and don't have my land legs back yet. At it's worst I have to hold onto things in order to walk, and I've fallen a couple times. Mostly it just makes me feel off balance and unsteady, and just a very weird feeling.
I kind of think it might be inner-ear related, but I'm not sure about the near-fainting episodes.