Finn
I had the same problem,plus a very rapid heartbeat. This was just one of the many symptoms of mercury poisoning that I had.
I had my amalgams removed 3 years ago and have been detoxing since.
The episodes diminished quickly. I would say they might surprise me once or twice a month; but only for a short while.
I think the reason they happen is that mercury is a neuro-toxin and it messes up our autonomic nervous system that controls the heart and lungs amongst other organs and glands.
The exercise must dislodge the mercury from somewhere-my guess is the joints-and then it attacks the ANS.
You might want to try taking some Algin before you exercise and see if that makes a difference. Also take a look at your tongue and see if it has a coating on it; or do you notice a metallic taste?
Hope this helps
Al