Mine did this and I fixed it. Open the battery cover, take out the battery and very carefully bend out the little contact that touches the side of the battery, so that it puts firmer pressure on the battery case.
My Garmin reported heart rates varying from 30 to 270 and stuck on a fixed rate for a long time. Actually thumping the strap while I was running along brought it briefly back to life which indicated it might be caused by battery contact. Since doing this, the chest strap has worked fine.
They are also susceptible to static from certain man-made fabrics. If the battery contact thing doesn't fix it, try a short run in a cotton T-shirt to see if it does the same thing.
Cheers,
Ian