Are you taking any supplements or prescription drugs? The timing may be key here if it's either one of those...when you take it, when it enters your system in a sufficient concentration, and then it has the effect. Or, it's an activity....maybe a diurnal/cyclic hormone like somatotropin, maybe its a wonky thyroid. There are literally scores of 'triggers' that people report and swear sets them off at least 60% of the time.
Have you had your magnesium levels checked? Potassium and sodium? They are key for proper heart rhythm.