ding ding ding..

I've seen interference from my jersey in the wind on a cold dry day (probably electrostatic) and power lines. Some coded hrms work better than others, the garmin's seem OK, my polar ALWAYS had interference problems..

FWIW, 220-age is BS, just throw it out. I'm 40, will be 41 next month. My lab tested max hr was "theoretically" 204, I hit 206 in a race last year and the lab guys believed it was realistic. My "HR @ LT" is 185 (done via LT testing).