Has anyone had this problem? 3-4 times in the past month since I bought my phone, my IPhone suddenly, without warning, turns off, in the middle of whatever I'm doing. The phone has been fully charged every time this has happened. It will not turn back on until a few minutes later. Why would this happen? If the phone is "defective," has anyone had experience with Apple's customer service -- how do they handle these type of issues? Thanks!