The phone has been away for repair; probably just had the firmware reflashed?
It sounds to me like an app is consuming system resources and causing a memory leak.
Android will reboot to safeguard the system if it's resources are at critical level.
Have a look at the power option in settings. The screen on time will show you if something is keeping the phone awake?
You can also try using the phone in "safe mode", this reboots the phone with only stock apps running, all downloaded apps will not run.
Long press on the power button
when the power menu appears, long press on power off.
You will have a new option to restart in safe mode.
If the phone works without crashing then you know it is one of the apps you have installed.
If the phone still crashes, it might be faulty hardware?
