I would suggest doing a manual check as Howsteep suggests. If your finger count is reasonable (130 for you?), and the HRM reads high at the same time, bike away from any power lines or factories. If it goes back to normal, I think you have electromagnetic interference from power lines or something. I've seen this problem with a Polar when riding near power lines. Not all power lines, but on successive passes of the same power lines. As an electrical engineer it does not surprise me (in fact I expect SOME interference with low-cost consumer electronics) at all if high stray fields are causing this minor disruption of the HRM, especially if it goes back to normal when you go away. If it's always reading this 216, I think you have a bad HRM. If its really random, I think you have a bad HRM.

Road Fan