Hi there, I read your post and had the exact same issues. After doing thousands of hours of research and having a few science degrees myself, i discovered what this is.
This is a nervous system disorder, it occurs when the electrical impulses or signals around the body react seemingly out of wack with your environment and it can be extremely unsettling. The next time it happens drink a warm cup of water with a few tea spoons of cayenne pepper for a fix within a few minutes. Cayenne will stimulate your nervous system at the same time as regulate it and relax any constricted arteries (from anxious stress). For a long term fix 90% of these cases are psychological, so usually when you read info like this and learn there are ways to control it and it is not actually going to hurt you, your subconscious will learn not to react to your conscious over analyzing every little thing that occurs in your body. The fact doctors are generally ill informed re all this certainly does not boost confidence. For the rest of causes, nervous system disorders can be due to medications / drugs / certain paints and chemicals and lack of certain nutrients and metals in the diet or an excess of certain metals in the diet. I know Blackmores do vitamin pill for Nervous system support that has all the correct balance of metals in it and it is mainly B vitamins. If you wish to slow heart rate quickly cold water to the face (and body if you can) then sit in front of a fan (it will slow instantly as your nervous system prepares to lower oxygen usage thinking you are under water).
A good way to fix the psychological aspect of it permanently is to do what goes against every bone in your body and when it happens go for a flatout run! try get your heart rate as high as you can. At first you will seriously scare yourself, but you will then realize thats it.. nothing comes after.. and what was once fear of a potential heart failure is now in the past. Remember the tests you had done are extremely capable of detecting even the smallest things wrong the fact that nothing went wrong was because your subconscious believed you were safe in the hands of medical staff.
!You know I would just like to say to all the people out there who go see multiple doctors only to be told that it is in their head; don't give up you know your body better than them...tell them to find it or give me my money back...I went to multiple docs over a three year period almost on deaths door literally could not get off the couch...dizzy, nausea, overall feeling of unwell...stools were not correct...just bad all the way around..was told I was crazy by one...had Irritable Bowel Syndrone by another and Anxiety by another...finally my ob doc...agreed to do a total Hysterectomy and low and behold after multiple MRIs and colonoscopy's and barium enimas and all the goodies he opens me up and freaks....calls in another surgeon...They do a resection on my colon...as I am filled with endometrion tumors in my whole abdomen cavity...one has almost pinched of my colon completely they think it is cancer (my doc was whiter than me) and I can tell they think Lawsuit coming our way...I had heart palps...dizzy...ab pain...just all kinds of feelings....so don't let these yahoos on this forum tell you not to follow your gut feeling...two months after my surgery...I did not sue..by the way..I don't believe in it.... I was on the happy train...I was kayaking, surfing and living life to it's fullest...!!! Good Luck and don't give up until you find a doc who will listen and understand and do your own research and take some vitamens too!! Can't hurt.....