At 75.5 I am trying to restrict my HR to <140 on long climbs with some peaks up to 150. Occasionally, if climbing at this HR for a long period 1.5-2 hrs my HR will jump to 160-165 on my monitor.
Definitely, this is not my actual HR. The monitor must be pickup occasion brief fast beats.
Furthermore, after and extended intense climbs as above, when resting, my HR will drop to 110-115 and stay there most likely indicating that I am over tired whereas after shorter climbs my HR will drop quickly to 90.
Maybe, the extended intense climb is causing a short arrhythmia.
As mentioned earlier, my cardiologist is against high intensity. Arterial pressure goes up with intensity, and that may cause serious problems. Of course, high intensity is relative for each rider, and everyone makes their own choices.