My first Garmin Premium HR strap started to exhibit erratic readings a little less than a year after I started using it. I had followed a basic protocol of rinsing and drying it regularly, but that's not enough for these fabric based straps. Based on some articles and information from Garmin, I developed this simple procedure for regular cleaning. This procedure doesn't use much water and doesn't require waiting to wash the strap with a load of laundry. By using this method, I've been able to reliably use my Garmin Premium HR strap for almost a year and half with no erratic readings.
The main cause of the erratic readings seems to be, as far as I can tell, the buildup of "salts" or other material between the contact snaps of the straps sensor attachment point. If this area has an excess of "salts" or other material that would make it conductive, it effectively "shorts out" the readings that the sensor is trying to get from the straps conductive pads.
Here's all you'll need to do a quick regular cleaning of your Garmin Premium HR strap.