Again, as a non-doctor and being unaware of all your circumstances, it could be that you have a longer than normal QT interval on your heartbeat. This is, I'm told, the reset rate of the heart after beating and if it is beyond so many milliseconds it can lead to periods of very rapid heartbeat which begin and end spontaneously. Once the interval gets beyond so many more milliseconds then there is a possibility of the heart failing to beat effectively, leading to possible collapse and worse. The sodium gradient across cells is also significant with regard to heart function so your salt intake is one more lifestyle choice that you may wish to consider along with the others recommended here. If you have the ECG and this interval is normal then you will have removed one of the potential problems from your list and can consider the others. Good luck.