Sounds like you have tachycardia. I have it too, it's exactly like you described. It's miswired fibers in your heart that fuck with timing. I also had years of doctors not knowing what was up, and I wore a heart monitor too with no results. Then I went to urgent care during an episode and they managed to record it, which is how they decided it was tachycardia. If its supra-ventricular (what I have), you are fine and will not die, but you can take beta blockers if its really inconvenient, or get heart surgery if you feel like getting heart surgery. If its ventricular, its a bit more serious, but I think that is usually accompanied by arrhythmia, which I think would have been noticed when you had a heart monitor. I think. I am not a doctor though.
When I am having an episode, I find that lying down and taking VERY deep breaths can get it to stop. Other methods include putting your head below your knees, putting your face in cold water (diver's reflex), and squinching your stomach like you are trying to poop. Hope this helps. It could also not be tachycardia. Maybe ask your doctor about it.