I get the flopping fish feeling and weakness. My rapid heart beats are usually regular, just way too fast and can be measured with my home monitor.
I've had to go to the ER when I couldn't get it to stop using Valsalva maneuvers or coughing. Once I had my heart restarted with Adenosine. Not fun, but it had been racing at 165 bpm for 5 hours.
Like cars, it never acts up when you're on a monitor.
Usually I can trace my racing heart to eating a trigger food.