Maybe you are hyperventillating?
Hyperventillation does not necessarilly mean you are anxious or panicking.
It can also be the result of breathing in too quick and too deep, and not breathing out slowly enough.
If you breath in more than you breath out, your body gets too much oxygen.
This can lead to get muscle spasms, high heartrate, chestpain, fainting...
but in healthy young people without heartdisease it probably is almost impossible to die from it.
Having a high heart rate for long periods is not healthy for your heart though and can do some damage.
The buteyko method teaches how to stop hyperventillating.
http://en.wikipedia.org/wiki/Buteyko_method
Can you describe these "episodes" a bit more clearly? Since the doctor didn't seem to give you a name for them, perhaps you can describe how the heart feels when it acts up. Then we could share our own experiences and information that we've gathered over the years.