sometimes your jersey flapping in the wind will give you readouts of the the vibrations as heart rate. in all reality if you are much over 200 you might wanna figure out its accuracy/see a doctor. this jersey flap problem is known for on the garmin straps. it is especially prevalent in the winter/dry climates. I ahve found my luis garrneau to be the worst.
to fix this problem I make sure to wet my chest and/or the strap before putting it on(with spit or water from your bottle) to strengthen the connection your sweat once your warmed up will keep the connection and/or wear a baselayer under my jersey to separate the jersey from my strap.