Palpitations only alleviated with strenuous excercise

I've suffered from palpitations fot twenty years now, but in the last 3 I've begun to experience skipped beats and very slow pulse as well. I've had an echocardiogram and I wore a holter moniter for a day, but naturally, nothing happened on those particular days, so I'm "normal".

Unfortunately, the light headedness, dizzyness, exhaustion and confusion that come with the palpitaions, not to mention, nearly blacking out make it hard for me to believe that everything's normal. I've tried eliminating caffeine, I do yoga, don't smoke, I'm fit and eat very healthy, etc.

My main question is: what does it mean if hard excercise is the only thing that seems to help my heart stay in normal rhythm? When I was pregnant, I actually had to keep up my running regime to everyone's dismay or I would be bedridden from the palpitations on the days I didn't run. Walking doesn't do the trick- I have to run! It seems like I have to jolt my heart into action or it will slow down so much that it just skips beats, then beats wildly to catch up, or sometimes even gets up to 180+ beats per minute for minutes on end. For some reason they get worse when my son is nursing too (he's breastfed and 20 months old).

Does the fact that it improves with hard exertion make it easier to determine what might be causing this?

I am on a thirty day monitor right now, that might be something to check out.