About once every day or maybe every second day my ICS upgraded SGS2 just dies. It takes a very long power button push or even battery removal/replacement to restart it. It did it just now when I rejected an incoming call. The worst thing is that I often don't know that it has turned itself off and it can sit in my pocket or on my desk for an hour or so. Then I wonder why the phone is so quiet, then I look and it isn't on I'm a complete amateur at the Android OS - How can I go about isolating the cause of this issue?