Hello
It sounds like you truly have had every test that is available for the evaluation of cardiac pathology. I agree that other sources of discomfort like GI should be explored. Additionally, I encourage you not to underestimate the contribution of stress to the development and maintenance of physical disorders. Often times, self reflection can reveal an increased level of stress that might prompt one to address this process.
It may be reasonable to see an electrophysiology cardiac export to see if there is some type of conductio-electrical disturbance in your heart system
In my book. Your Guide to Optimal Health (www.personalwellnesswheel.com) I elaborate on the numerous physiologic changes that occur with increased stress including elevated blood sugar, increased cortisol, blood pressure and arrhythmia. Stress is not benign.
It does appear that you are doing everything within your means to live a healthy lifestyle and I congratulate you on that and encourage you to continue. Good luck in your quest to identify the cause