Irregular heart rate and dizziness when walking up steep grades

I have had frequent episodes of an irregular heart beat when quickly standing up and walking up a flight of stairs, when walking up step grades and sometime for no reason. The symptoms seem to originate in my abdomen and quickly rise up to the symptoms I described. I recently completed a nuclear stress test with negative results. The results indicated I had no ischem problems and tested above average. I am an 83 yo male. My doctors aren’t offering any diagnosis. So looking for other’s experiences. Thanks Jim