I've got the same, heart rate is pretty hit and miss, half the time it looks good and the rest of the time its nonsense.
I've found similar things with my Polar often I'll get a week of good data then several days of rubbish stuff. I've found properly wetting the strap and regularly cleaning the strap with soapy water gave much better results. But still not 100%
My watch has a permanent heart rate monitor on the back not a strap. It actually seems ok for running. Kayaking seems to cause it some major issues though or maybe I just work very hard in the boat!!