I Tend To Have Random Episodes Where My Heart Begins

I tend to have random episodes where my heart begins to race for no reason at all, my chest begins to hurt very bad and my head starts to hurt and I feel nauseous. The episodes used to only last 30 minutes but have now increased to last over an hour. I've been to the hospital before but they found nothing structurally wrong with my heart and because the episodes are random, it is impossible to accurately record them. I am not quite sure what these symptoms mean but it is beginning to worry me because when I am having an episode I find it difficult to carry on with my normal daily routine.