Like many other people, I replaced my inconsistent, (and subjectively very uncomfortable) Garmin soft strap with a Polar WearLink version, and the random HR recording problems vanished (plus it’s not cutting into me like the Garmin version did, but maybe that’s just the odd shape my ribcage 😉 ).
I had a initial surge of panic the first time my HR rapidly rose to over 200, but as I’d only been out for maybe 20 seconds, and err, felt fine, that panic quickly passed, and it went completely when my watch carried on showing >240bpm with the HR strap held in my hand, then with it hung in the hedge behind me. 🙄
Spit helps too, as does getting/keeping the sensor strip clean after use.