Were you wearing a shirt made of a technical material like KoolMax? There’s something known as ‘flappy jersey syndrome’ where loose fitting, technical shirts cause static electricity and upsets the HR monitor. Heck I’ve seen double my actual HR putting it up into the 240s. Once I start working up a sweat, it goes away. Also wearing tighter fitting jerseys helps.

I’ve never noticed the problem while wearing cotton t-shirts or using cheaper heart rate monitors. Come to think about it, I’ve never noticed it while running with my 305. However I’ve seen it while cycling with my 305 and Edge 705. The higher speeds might cause the jersey flapping action to be more pronounced.