Plax - I had the same computer with exactly the same problem... changed all the batteries, even tho the little green light was coming on, but still temperamental.
This was my second wireless computer and both had lasted less than a year... LBS replaced the first one but it took a couple of weeks after they sent it back to the suppliers. I couldn't be bothered to return the Sigma and just replaced it with a wired Cateye Mity 8. So far so good.