I recently picked up the heartrate monitor from MapMyRide. On sale for $49 seemed like a good deal.
It isn't. In fact, its junk.
I've used it three times and the readings are all over the chart. Doing a climb today it said my rate was 135...which is fine. But then cruising on flats it showed a rate of 224. It frequently shows a rate of 180-190...while I'm simply circling the parking lot.
Very high readings like those are often due to static electricity when the jersey is flapping in the wind. Google "flapping jersey syndrome". Some riders have wondered if they should call 911!
It's more common in dry conditions, too. Next time you see a reading over 200, try holding the jersey away from the strap for 10 seconds, and see if that helps. Or stop riding, and see if it goes back to normal.
Try wetting the contacts before riding with a drop or two of water, and move the strap a little higher on your chest. Some people have used dryer sheets to limit the static electricity, with varying results.
My Garmin hard strap has been very good, I haven't seen the high readings at all. My old Polar used to do it occasionally.
^^^+1. I put a few drops of water or spit on the contacts and rub it in. Be sure the strap is reasonably snug and the contacts located on the ribs just below the pecs. Snug jackets and jerseys that don't flap can help. Some suggest spraying the offending items with Static Guard, but I haven't tried that myself.