It's likely "flapping jersey syndrome". Static electricity confuses the HRM. I see this more in the winter, when it's dry. My HRM will jump from 140 up to 220 in a few seconds. If you see this, try pulling your jersey away for 10 seconds to give the HRM time to update it's averages, and see if it goes back to normal.
Also, I have to put the strap quite high on my chest to make it accurate.