I am having the exact same issue for the past few weeks. Phone cashes then shuts off, sometimes while Im on a call. It usually begins to act up when my battery is at 30% or less. When I try to turn it back on it wont come on unless I plug in. I have noticed too, when my battery was not low and it shuts down, when it comes back on the battery says 5% percent, but quickly charged back to the percentage it was at within minutes. Muy frustrante.