This is a common problem on heart rate monitors - mostly happens when experiencing dry headwinds or cycling at speed (downhill etc). Caused by static electricity from cycling shirt rubbing against the sensor. Had this on my Polar and Garmin Edge 305 as well. Seems like it has been sorted on the Garmin Edge1000 though. When it happens again, press against your chest where the sensor is - stopping the rubbing of the shirt against the sensor, the heart rate figure should drop back to normal (until you release again :-)). I also found that it does not happen with certain types of shirts - such as winter garments with more wool woven into the material.
