rachel69, I know just how you feel when these episodes occur. I remember when mine first occured they really stressed me out, causing major panic and anxiety attacks that I was going to leave this earth! Unlike you, my cardiologist, a very prominent and experienced one, told me that my palpitations, occasionally racing heart and "internal tremors" were benign and anxiety related, and this was just from listening from my heart and an ECG/EKG! She told me that it is highly likely that it would be a waste of money for me to do further tests such as a 24hr holter monitor or an echo, so I didn't pursue the matter.
It's now about 2 months after the palpitations first started, and I'm beginning to learn how to manage them better. I still have bouts where I am panicky and anxious, but with some self-assurance and anti-anxiety medications (I take a low dose of Ativan when needed) these symptoms have been easier to manage. It does take time for all of us, but first it has to start with us trusting our cardiologist's assurance that nothing is wrong with ourselves based on our test results! It certainly doesn't take overnight, and even those who have learnt to live with our condition are still subject to the occasional attacks.
Personally, when these happen to me, I'd find something else to occupy my mind - an idle one is a dangerous thing! I'd call friends up for a chat, for example, or read a newspaper and try to remember what I read later to focus my concentration. I'd do things like coughing hard, drinking water, blowing my nose while pinching it, and if I'm up to it, do some exercise on my treadmill. I find that while these may not work all the time, they do work some of the time, and certainly preferable to taking medications on a daily basis! I also have hypertension which certainly doesn't help!
I hope this helps in allaying some of your fears about your own situation. As suddenly as these symptoms appear, they may just completely disappear if we're mentally strong enough to manage and ignore them. At least that's a hope that I harbour on a daily basis!