Oh my GOODNESS!!! I have tried and tried to explain this feeling, as I have it too sometimes!! It's so scary, and the main thing is (and you put it VERY well), I feel like I'm going to pass out unless I concentrate on something! I can't believe this. I thought I was the only one.
I have mostly had these "episodes" at night, but there was a period of time, about 3 years ago, that I had them during the day as well, for about 2 days. I was scared to death, as they were happening while I was driving, and I was afraid I was going to pass out if I didn't concentrate. I ended up going to the ER, and they did a brain scan (CT), and they said that my sphenoid sinus (in the middle of the brain) was "thickened" from a sinus infection, so I took antibiotics, and I felt better.
I've tried to make connections, and what I've come up with is this: they're more likely to happen if I'm sleeping under a fan and my sinuses (the sphenoid drains directly into the throat) are acting up (mainly just drainage in my throat). I've also wondered, though, if these are some type of seizures. I've read about the "aura" that happens right before a seizure, and I'm wondering if that's what I'm feeling when I get that weird feeling...but it's like it never completely happens...I just stop it by concentrating or something.
I really wish I had more answers, but I'd like to find out more so that we could figure this out together!

I'm sorry you're going through this, but we will get some answers. Do you sleep under a fan? Do you have any drainage into your throat? I'm just wondering if the "sphenoid sinus" could be inflammed from a sinus infection, as it gets thickened and presses on parts of the center of the brain. Are you on any meds? I take
hydrocodone for chronic pelvic pain from endometriosis/adhesions, and I've wondered if that could contribute ever? We'll get this figured out...maybe by making connections about our environments and our symptoms, we can nail this down! I'm praying for you!