Comment by invalidptr

This is unfortunately very common for OHRMs. Once your heartbeat approaches your running cadence (~180 bpm), physical movement becomes confused with the color change of your blood that an OHRM relies on. Tightening your wrist strap can help, but if you want accurate heart rate data for workouts, a chest strap is the way to go. Garmin makes a very good and very inexpensive one.