Thanks for your health concerns. When palpitations are persistent and there are no evident causes, doing a 12 lead EKG is like trying to understand a movie from a still frame: you just wont have enough information. Therefore, in your case there might be further investigations such as a Holter monitor
(ambulatory EKG) or an event recorder to try to "catch" one of those episodes and see if they are related to an abnormal rhythm of the heart. You can discuss further with your local cardiologist.
Hope this is useful, wish you the best health,
Dr Brenes Salazar MD
Mayo Clinic MN