I have this too, from time to time, bubzoo - I've had all the tests and there is nothing wrong. It's quite scary when it happens and I know what you mean - it makes me feel a bit giddy too. I find if I cough, it seems to set it right again. Scary when it happens, but nobody seems to know why, although It does seem to happen to me when I have a full stomach - someone suggested a connection with the vagus nerve, as it runs through the body. I've learned to live with mine, but it's not nice.
OG is right - the more you think about it, the more you notice it...