I am suspicious that it is a hardware problem because my unit has been fine for the past 2 1/2 months. I am running stock, unrooted and was without a case until two days ago. I've made no recent changes to apps or settings. The only change was that put a bumper on and my phone has suddenly started shutting down and remaining unresponsive for a period of time. The battery is always at least half charged (before and after the event). After it decides to come back on, it is as if nothing happened. It doesn't really strike me as a software issue, it is more like the battery suddenly disconnects.