Tachycardia Long After Exercise

Hello. I am a healthy 53 year old male. Frequently during exercise, I experience tachycardia at about 160 BPM. It only slowly drops and is still 120-130 hours later. I have seen two cardiologist who ran stress tests, holter monitors, echos, scaling cta, etc. Every test showed a healthy heart. My Duke score was 11. The only explanation was that my heart was "reacting" to something but to what we can figure out. Has anyone suffered anything similar?