I also have found the Suunto heart rate chest monitor to be inaccurate. In my case, what usually occurs is that when I increase my exertion, and my heart rate increases, my watch displays an increasing heart rate, often to above what I think is my maximum heart rate. It just climbs and climbs. I changed the battery and religiously cleaned the strap. This did not rectify the problem. I then bought a chest strap monitor and wrist display from ebay for thirty-some dollars, and it appears to be accurate, based on checking it against my carotid pulse. The brand I purchased on ebay is Ovente.
I wish that I had not purchased the expensive Suunto Ambit 3. I have contacted Suunto, and they have not been helpful.