I signed up to respond here. I get this too. It's horrendous. I call it the "kick to the head" and it feels like a mini stroke or something. Can happen at any time, can take several hours, days weeks, or at its worse, a full year to recover. Sometimes happens in my sleep.
I'm getting a carotid artery ultrasound soon, and while I don't want them to find anything, it would almost be better to know WHAT is causing this. It's plagued me for several years.
I also have other POTS-type symptoms. A friend of mine has POTS very badly and said I sound like a classic dysautonomia case so I'm looking into it.
