I'm getting an identical issue to this with all the same background info, totally stock phone - back in late 2012, on O2, no dodgy apps.
Seems like it could be SD Card, Chrome, or screen ripple (although not sure on this one!). I'm on 4.1.2. and had no problems, and thought it might be juice defender as someone else mentioned 'task killers' so I removed it. But when I reboot by holding Power Button down as its the only way, it sometimes reboots but still in a locked/frozen state.
Happy to try suggestions, but some of the options are like literally taking a punt, e.g. uninstalling all apps, etc, Some suggestions from Samsung, Android would be good!
Getting fed up with Android, having been an iPhone user, the OS and hardware just don't seem as stable I'm afraid.