How fast is your "racing heartbeat"? Have you measured it? Does this come on and build slowly, then slowly subside? Can you give us some more detail about the event itself?
It is 120 BPM and it comes on slowly. When it happens it doesn't stop for a few hours, but it subsides slowly. It's worse if I am doing something active, like at work. At work I can't stop what I am doing, so I have to work while it''s happening and the racing heart, sweating and feeling over-heated makes me very exhausted. Sometimes it is accompanied by sharp pains in the left side of my chest right where my heart is.
I'm sorry, but I really don't have an answer for you. I had hoped you were going to describe a "racing heartbeat" as 240bpm, and that it came on suddenly. THEN, I'd have some advice for you. But a rate of 120 isn't really all that high, especially if you have an active job. Perhaps not lying prone though. Are you sick? Do you have an infection? These are things that could possibly raise your heart rate. I'm sorry that I'm not of more help to you. Bumping this up to the top hopefully will result in additional responses.
It could be a bit of dehydration but if I were you I would push to see a cardiologist and get a 30 day monitor to catch what is going on. the chest pain and sweating to me seem to be enough to warrant further investigation. You say you have seen 3 doctors, were any of them cardiologists? Even if they were and you have had an echo done to evaluate the general health of your heart I would still push for a 30 day monitor to see what is going on. 30 day monitors are better than the one day monitors for symptoms that don't happen daily. Best of luck. Hope you can get to the bottom of things. Keep us posted.