A friend of mine was "diagnosed" with chronic fatigue syndrome and spent quite a few years hardly working. Sick of it he went to trade school on weekends for something new to learn , and whilst walking up some stairs carrying some timber got some chest pain and had a near faint.
Paramedics called - ECG done - pt had actually been suffering Wolfe-Parkinson White syndrome. Look it up on Google for a detailed explanation.
He underwent a procedure called an ablation to the area of the cardiac tissue that was causing it and bingo ,problem solved.
Keep pestering your doctors and cardiologist.