Did you doctor do a complete metabolic panel? I was having similar problems (except they happened mostly at a resting rate) and I had a battery of tests. I felt like a research experiment I had an ECG, Heart Monitor, two metabolic panels, EKG. I saw my internal medicine doctor, a neurologist and my eye doctor. They have determined it is most likely a form of migraines. Not the typical type, but something like it.

Anyway, I am just trying to tell you the path I had to go down to get some answers. Like I said it was similar, not the same. An echo doesn't seem like a stopping point though.