My normal heart beat is 72, when i do aerobic exercise i get it up to 130 but twice during the exercise and aftere approximately 25 minutes it had suddenly dropped from the 130 to about 75, quite regular though at 75, it happened once whilst bending and once doing crunches. Pacemaker is set at 60 so thats not in play. Both times i have stopped the exercise and gone into cool down mode.
Do I read correctly that your HR drops when you begin to exercise? 120 at rest, 75 when exercising? This seems very strange, but if true I have no comment/experience.
There are several members of this Community on pacemakers, perhaps one will yet be able to relate to your post.