hi~~ you are really on top of things, and definitely a model to many people who'd prefer to hide from their symptoms. you have hit these issues head-on, and i applaud you for it!!
here are my thoughts/suggestions... maybe one of them will be helpful:
1.) could be something more like an upper respiratory infection
or virus. has the doctor done any chest xrays? may be useful, as heart and respiratory problems really can mimic each other.
2.) go to another doctor. a second opinion
never hurts. if nothing else, try going to an urgent care clinic in your area. at the very least, the other MD would be able to repeat tests and maybe have a fresh perspective on your symptoms.
3.) ask for a referral to another physician or to a specialist. 4.) how's your blood pressure? fluctuations in blood pressure can really do funny and odd things to the body. these fluctuations would probably be sudden, and would not necessarily be detected on a routine doctor's visit. pick up a monitor at walgreens and chart your results. you may find a correlation between your symptoms and your BP.
honestly though, you have been doing everything just right. it can not hurt to express these concerns to your doctor and be persistent if you are truly worried. one positive thing here it that you really have been thorough about your tests and have had very positive feedback thus far.