https://www.mayoclinic.org/diseases-conditions/enlarged-heart/symptoms-causes/syc-20355436
Did your cardiologist order a Holter monitor for you to wear for a week or two/three? It may be that you have an occasional arrhythmia. I have had two MIBI nuclear stress tests on treadmill (both negative) numerous X-rays, angiogram, ultrasounds, Doppler ultrasound, echo, several ECGs....none of them showed any significant deposition of plaque. It wasn't until I had a polysomnography, an over night sleep lab, that my severe sleep apnea was diagnosed. That was the cause of my AF. I'm telling you this because there might be other diagnostics needed to pinpoint your problem, which might be sleep apnea, might be an arrythmia, maybe both...?
I think they'll get it solved, but you may indeed have to go further afield for an opinion.