My average heart rate is always 150 and my max heart rate is always 161 on every single 4 mile run regardless of pace or effort. For 6 miles the readings are 155 and 161 - every time. This has to be a malfunction of the monitor. The graph shows the heart rate going up in a straight line until it plateaus at 161 even on runs where I am flat out for the last mile! Is there a fix for this?