Not sure if this has been covered before but thought I would share my experiences with my HRM . I changed from a polar HRM device to a garmin around Feb time. It recently I've been getting quite a few incorrect reading. It seems to spike to around 240bmp for a little while then slowly settle down again. It didn't seem to do it with the polar. I decided to get some electrode gel. A small drop of the gel on each electrode really helps with the reliability of the data and it almost never spikes now (only if I move the chest strap around ). If you are having problems with your HRM then I suggest you try some electrode gel (not sure about availability in Japan though, I got mine in the UK).