I wonder if you might be deficient in either, or both of, magnesium/potassium. Both are critical for heart function, and some of us seem to be off the marks, either high or low. It might be worth a look. If you get agreement from a cardiologist, he should want an RBC level indication, not just serum levels...in the blood. You want to know how much of it is being absorbed and used.
It pains me that you seem to get no relief or credibility from a series of specialists. My advice would be to continue to look, even to travel if you must. Sometimes teaching hospitals affiliated with universities are the best places to go.

Also, and I hate to say so, but there are times when you'll get the best diagnostics at an ER. Keep looking for a sympathetic cardiologist, but in my experience an electrophysiologist is whom you need to see to get the very best care for a heart with palpitations.