Have you had an EKG? That gives a picture of how your heart's "electrical function" is doing.
I had a similar situation. I went to the doctor for something else, and on the intake interview, they found my pulse to be 36, blood pressure was normal. They did an immediate EKG, found that my heart was "misfiring." They were delays between beats, and missing beats.
They sent me to the Emergency Room. The very next day I had surgery and a pacemaker implanted.
Before surgery, first they checked me for thyroid function, and also checked to see if I had had a heart attack. The answer was no to all that. I have no heart disease, no cardiac disease, arteries are fine, cholesterol levels fine, heart muscle is fine, thyroid fine. The only problem was that the nerve nodes that propagate the signal to contract the heart muscle were dramatically failing. A pacemaker fixed it right up.
So, good luck and best wishes to you. You might consider a visit to a cardiologist, just to get an official expert opinion.