I've had this off and on for a LONG time; had treadmill stress test, Holter monitor, EKG, etc. on multiple occasions.   I'm 57 now; I've been passing flight physicals for years, both USAF and FAA examiners. One flight surgeon even commented on it. Absent any other heart problems, he told me not to worry about it.   On my last physical, my PC doctor sent me to a cardiologist, considering my age, just to be sure. He (the cardiologist) calls it the "curse", as it presents in so many people. EKG and Holter have me a clean bill of health.   I do notice if occurs more often after a sustained periods of stress; it is scary, or at least annoying, but if your Doc's don't find anything else wrong with your heart or thyroid, or elctrolyte balance, try not too worry about it.