Homesic, I think it would be a bit unfare to try and assess your condition over the internet.
It might be worth you getting a futher clinical assessment. The tests you mentioned wouldn't always pick up a problem, if one existed.
Heightened awareness of the heart beating, or an awareness of a change in beat, is quite a common problem, and it can be "normal" (ie no disease), or pathological (ie an underlying condition causing your symptoms, such as an overactive thyroid gland.)
It sounds like your clinicians have decided that the cause of your symptoms are non pathological, but in view of your anxiety about these symptoms, consult further ! (Even if its for reassurance only!)
Incidentally, the heart is a muscle in its own right, capable of beating autonomously from the rest of the body,but there are certain factors that influence it, either through the nervous system, (eg "stress") or chemically (eg nicotine, alcohol, caffeine.)
Hope the info's useful.