As I undertand it, sometimes friction with the jersey or other things can create some static that might affect the wireless HRM. I never happened to me with my 305 over 3 years, but I've read of others with the problem. If you get a good connection with the skin on your chest, like with an electrode cream, it should work fine.
Or maybe your heart rate is really abnormal and you should see a cardiologist to avoid sudden death.
It's probably one or the other.