I'm having the same problem. I use my HRM with a Garmin 350 and a Garmin 500, and I get the same erratic results on both, so the problem is with the HRM. But which part: the module or the strap? I tried a resistance measurement on the electrodes of the strap, and did not see anything that looked wrong.

I tried contacting Garmin Tech support to no avail. I get logged into the site ok, and I get to the support menu ok, but when I click on "Correspondence with Support" I get a blank page with some techy jibberish on it. So, no tech support from Garmin, for whatever reason. Maybe they have a software glitch on their site, or maybe they're just sick and tired of dealing with peoples' problems with Garmin devices.

If you can get thru I would be interested to hear