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I get high readings from Garmin straps (all models) if I move the strap around whilst wearing it. I notice this more when I'm playing 5-a-side as I tend to adjust the strap more.
I've also get sustained periods of high readings (2 minutes of 240bpm+) when passing close to buses or, oddly, one of the places where buses turn around (even if there are no buses there). I'm guessing there's some electrical interference that's affecting the strap of the communicatons from strap to Garmin.
Yeah, old plastic. From 2009 i think.