Similar issues
Since u have never flashed any rom yet and ur phone still has warranty, why dun u sent it for Samsung Service ??
Why worry so much ?
I faced a similar issue, where my SGR used to turn off when playing games, later similar scenes, that it will work without the sim card for few minutes, I had infected Customer rom, Flash Stock rom, took it to the service center, 4 days so far, they said they need to get the motherboard replaced, But i must tell, I ve dropped the phone lot of times before, But thrice on the day when it first shut off. Waiting for the motherboard replacement, Hopefully everything works fine after this.