Well, i reversed the polarity captain and it seems to have done the trick.
The powertap strap is a soft strap so it may well fail anyway, I'll keep an eye on the situation for now, maybe see if I can get a Garmin plastico cheaply somewhere.
I suspect there is not much to separate the models in reliability but obviously haven't tried them all (just Garmin and Polar). They're probably from the same factory.
Try taking the battery out, reinserting it upside-down, closing the cover (holding it on for 20 secs).
It sounds like I'm pulling your leg but this can reset by (apparently) reversing the polarity. It's worked for me a couple of times.
Also wash the strap. Salt is conductive (yuk).