My son aged 18 has been getting this, coincidentally always on the same climb on our regular midweek evening road thrash, I’m guessing because he’s been riding hard for 15 minutes by then and his glycogen levels are dropping.
Google carotid sinus massage; one of our cycling buddies is a physician and he has performed this twice on my son, which has stopped the palps immediately.
GtiJ has had the 24 hour ECG monitor thingy but of course no palpitations happened during that time. I used to get them a lot in my teens and twenties too.