I have an older Garmin HRM1G that came w/ my forerunner 305. I changed to battery out last xmas time frame. I do think it's working, the club ride we had on the 4th of july was hard and was in the upper 170s a number of times on each climb trying to keep up and/or limit how much I get dropped.
I do have the autopause set up at 2.5mph or slower. But on the main climb, I stopped once at the bottom to roll up my soaked windbreaker and pocket it. Strava is my stop watch, it even counts both moving and non moving time.