Also: To the best of my research, historically, Garmin had a standard "plastic" HRM strap, and a fabric "premium" HRM strap.
The initial fabric strap had a lot of problems and widespread complaints due to erratic readings due probably to static electricity and poor conductive patches. So many users downgraded to the plastic strap and reported that it worked much better.
Later, however, Garmin upgraded the "premium" fabric strap with better conductivity and electrode areas. So it is reported to have "fixed" that problem. The new strap is distinguished by the part number, the designation "HRM3" and has some obviously "thicker" (conductive ?) gray fiber areas near the electrodes.
This happened to me. I had problems with the old fabric strap, so I got the newer model. The newer fabric strap works fine.
Soft Strap Premium Heart Rate Monitor
Part Number: 010-10997-07
You still need to have good batteries and wash the strap regularly. Body salts/oils create a non-conductive layer on the rubber electrodes. Put a bit of laundry soap on them and scrub lightly with a soft bristle brush. Rinse I dry.