If I remember correctly.... The very oldest PM2 monitors were not designed to deal with "Coded" Polar monitors. Coded monitors send out an extra pulse every heartbeat, this causes the HR to be high by approximately the amount you are seeing. A replacement PM2 or a new PM3 or PM4 would solve this problem, as would finding an uncoded Polar strap.
Thank you. going back to an uncoded Polar strap worked for me. It was starting to drive me crazy as I knew what was displayed was wrong, but I could never tell for sure how much.