I, too, would be very interested to hear the results from your next tests. The symptoms you describe are very similar to the ones I have + I have noticeable premature beats at different times in the day.
Would you say you had a stressful job? Mine is pretty stressful and I haven't had a vacation for well over a year, and it's really wearing me at the moment. Sometimes I'll be completely calm and relaxed, enjoying something, then suddenly and for no reason I can feel a change inside me which is kind of a cooling feeling and some chest tightness. This then turns into clear anxiety, and for absolutely no reason.
It's pretty interesting really, as this has happened quite often. I don't feel any stress at all about it now, I just observe and feel these strange changes.
I made a series of lifestyle changes that really helped to minimize my symptoms, but I can't say I'm completely rid of them or their unpredictability.
I'll be turning 40 this year. I've heard it suggested that this kind of thing is just the natural aging process, and that when it happens it's worrying, so it makes the symptoms worse.
I'm taking a vacation this Sept. and quitting my job, so I'm going to see if this cures the problem.
We'll see. In the meantime, good luck with the tests. Try to avoid medications if possible, especially long-term ones.