Here is a quick fix for issues I used to have, and have seen others reporting, with Polar H10 heart rate monitors. It relates to random drops in heart rate during an activity, as seen towards the end of this long steady run;
When the strap was new it worked perfectly for a while, but then I’d start to see these dropouts in every run after a warm up. In an effort to resolve it I had tried replacing the battery, but would end up having to replace the strap, only for the same problem to return within 3 to 6 months. Others have said that this is what they do, claiming that the strap is a consumable item that needs regularly replacing, but that seems wasteful and doesn’t sit well with me.