Last thing that I want to do is alarm anybody, but it is possible that the heart monitor is working properly - and that your heart rate really was 218 for a moment.
One of the most important things about heart monitors is that they provide information relevant to how hard you and your heart is working. From the information that you receive from monitors, you can then modify your workout to maintain your target heart rate (THR). Conventional thinking is that when you work out, your THR should be 75%-85% of your maximum heart rate.
Your maximum heart rate is determined by subtracting your age from 225. For example, I'm 56 y.o., so my maximum heart rate is (225 - 56 =) 169 My THR range is 75%-85% of that or 126 (75%) to 143 (85%). So, when I workout, my THR is between 126 - 143 beats per minute. Depending on my level of fitness, I might be able to workout comfortably with my pulse in the 140 - 150 range.
But, for a 50 y.o. the Maximum heart rate is 175. What happens sometimes when someone consistently exercises at or near their maximum heart rate is that the heart delivers a few extra beats - called premature ventricular contractions (PVCs). When that happens, the heart monitor picks up the extra beats, calculates them and shows a pulse of 218 or something like that. Because the PVCs occur infrequently, the very high pulse over 200 can be mis-interpreted as heart monitor error and attributed to some of the things that it's been attributed to in this thread.
I'm not a physician, and I don't really know what the problem with your monitor is, but I do know that what you described can occur because of abnormal heart beats or PVCs. And, if you're posting this in the 50 plus forum, then your maximum heart rate is in the 170 range - higher than the American Heart Association recommends for working out.
More about THR can be found here:
http://www.americanheart.org/present...dentifier=4736