This is probably not what you have, but I wanted to tell you anyway: a colleague and friend of mine, an extremely fit 63 yr.old who's done long-distance sports his whole life, has had heart palpitations for many years. During exertion his heart would suddenly go into "hyperdrive", and he had trouble getting his pulse down again. Turns out there was some nerve or something "backfiring" and giving his heart too many signals. It was solved by a very short operation where they burned over the offending "signal" (sorry, I wasn't paying attention to the medical details ) and he hasn't had a heart palpitation since. Just FYI.