I'm having a problem where my 305 gets stuck at certain values and doesn't update appropriately. You can see it here on Garmin Connect; scroll down to the heart rate chart:
http://connect.garmin.com/activity/67527648
http://connect.garmin.com/activity/67197438
Notice the flat spots. By comparison see
http://connect.garmin.com/activity/66353661
I can tell when it's working properly because when I look at the watch the heart rate is always changing slightly. When it's not working it's stuck at some value.
I've tried tugging sideways up or down on the chest strap and that seems to get it going again.
I've tried shaving my chest where the chest strap is but that hasn't helped.
Any advice?