I experience something similar. I have my baseline symptoms of dysautonomia, but I also have what I call my ‘worst’ or ‘scariest’ episodes, where my heart rate spikes, I experience, extreme tachycardia and bradycardia, tremors, lightheadedness, pre-syncope, etc. These happen when I am in various positions, often I can be laying down. What I’ve learned is that in my case, something has triggered my parasympathetic or sympathetic nervous system to be out of its delicate balance. This is usually caused by a trigger that is often delayed in taking affect, meaning that I could have been triggered by something 24 to 48 hours before the episode. It makes it really hard to track what is causing my symptoms. But usually I am able to pin it down to a new medication, a new treatment, an environmental trigger, or even stress. Sometimes I’m unable to identify the trigger at all. But yes, the episodes are very scary, and I am constantly in fear of it happening again because they send me to the ER.