From a totally different angle: I used to have similar "episodes". I even had them while falling asleep, and they made me jolt fully awake.

In my case it turned out I was chronically hyperventilating (which is mostly harmless). Not saying that's what you are having; just something to keep in mind if the MRI and tests don't provide a clear cause. Stress & hyperventilation can provoke the weirdest sensations, and contrary to the comic strip depiction of it, it can be very subtle.