My 4S has been dying at random lately, no matter how charged the battery is. When I try to turn it back on, it instructs me to plug it in. If I do, it turns on immediately, but if I leave it for 15-30 minutes it'll turn back on like nothing happened. This happened three times last week and twice today, all while using either the camera or foursquare. Does this happen to anyone else? And does anyone know if it'd be a hardware or software (iOS 7) issue?