I have a Forerunner 645 (non-music) and recently got a HRM-Run strap to use with it.
The setup works fine for normal running, but when doing intervals I've noticed the HR data drops out frequently during the walking rests between the intervals.
I'm guessing that this is due to the different arm position during the walks, i.e. the arms are hanging straight and the watch is therefore lower down by the side, with the signal between the watch and the strap being intermittently blocked by the torso as the arm swings, resulting in the dropouts. I've swapped the battery for a new one with no improvement.
I got in touch with Garmin customer services about this and they said the unit was faulty, so I sent it back to Amazon, got a refund and bought the HRM-Tri instead (same price - I realise the data caching of the Tri will not work with the 645). I've just been out to test it and exactly the same problem occurs!
Is it unrealistic to expect an uninterrupted signal between the strap and the watch if it has to intermittently pass through the body, or could there be a fault with the 645?