Problems with Sigma HRM

suffolk rider

I bought a Sigma PC15 HRM last year, and it worked very well for about a year. But in the last few weeks it has started to cut out inexplicably (and then equally inexplicably cut back in), which is a real nuisance, as it monitors maybe only 3/4 of each ride. I thought it might need a new battery: the watch seemed to work fine on its own so I replaced the battery in the chest band, but the same thing happens. Has anyone else had an experience like this, and if so can you suggest how I can get around it?