If your heart rate monitor strap is not in extremely good electrical contact with your chest, it will generate spurious measurements as it rubs or bounces against your chest, giving artificially high readings on most monitors, or causing the monitor to "hold" on the last good number it had, even while your heart rate has risen or fallen. This is particularly a problem on cool, dry days when you won't make enough sweat to establish the contact, and on windy days when a loose jacket flaps against the strap, making it bounce against your chest. The solution is to improve the chest-strap contact, first by making sure the strap is tight enough and then by adding a contact enhancer. At the doctors office they use "contact jelly" with ekg machines. "Personal lubricant" (one popular brand is "KY Jelly") from the pharmacy aisle of the super market or drug store is essentially the same stuff. Put a good sized dab on each contact patch of the strap and the strap will maintain contact even as it shifts around or bounces a bit. Rinse the strap after each training session to avoid a messy build up.