Have the same issue. You've probably already found out it's called Cardiac Drift. I can run at the same pace for a 5k, and my heart rate is linear up to max/max+ every time. I ran ~500mi last year and it was the same almost every run. I tried to slow down from ~8:50mi to ~10:00mi with no real discernible difference. Kind of a bummer. Going to get my arteries mapped out soon (former smoker).