What allows arrhythmia and utter weakness to abate over a period of days, only to recur, again, after protracted strenuous activity?

Arrythmia & Fatigue. It sounds as if a full workup and evaluation is necessary including a good history and physical, ekg, and basic chemistry and endocrinologic/hematologic profile. This is only the beginning. It is likely that further follow through with an event monitor/recorder will likely be required. See a physician soon.