I've also had problems with heart palpitations since my teens. I've also had several episodes of syncope (fainting). I've had a full work up and everything came out A-ok. The only thing is my BP runs very low, but that wouldn't cause palpitation. However it can cause light headedness and fainting) I've learned to recognize my symptoms (light headed, nausea, tingling in my arms) before I pass out, and I can sit or lay down.
As for the palpitations, when I completely cut out caffiene from my diet, I do notice an improvement. If you haven't already, I suggest you do the same. Start keeping track of when you get them, and write down what is happening. Are you exercising? Did you just stand up from sitting? Did you have a lot of coffee earlier? Are you stressed? This way when you go into to see a specialist, you can give the doctor some clues.
Sometimes, people do have symptoms, and after all the standardized tests, there's no sign of a problem. But you should still have all the tests to be sure. That would be up to a cardiologist to decide what you need, and writing down the issues around you palpitations will help him or her get a better idea of what be going on and what to look for.
If you've been to doctors (you said in your OP) and they have found nothing, there may be nothing to find. Sometimes the body doesn't work 100% perfectly, and most people at some point experience palpitations. If all your tests were normal, then the doctor won't have any diagnoses for you. Unfortunately, that happens sometimes. If you feel more comfortable, you can always go see another cardiologist, for a second opinion.
I hope this helps.
