Power lines and even traffic lights can affect a monitor- but the most common cause for me was riding in close proximity to another rider that also used a heart monitor. Cross- readings will occur on some of the older Polar models and cheaper units. We had the problem on the Tandem as both of us used Polar units. Even with the higher priced models where cross interference is not supposed to occur- we had to ride with one strap on at a time between us. It was good for me as if I was wearing the strap- my pilot could tell when I was about to cry enough- so he always rode within my limits- just.

I changed to a garmin and never had the HR register problems- But Elevation spikes and speed spikes occur occasionally. Normally near power lines.