Phone keeps shutting down

I have LG Nexus 4 16GB version. And it is shutting down every now and then by itself, specially while using any Google application. (eg: Google Camera, Chrome, )

It just shuts down. And I have to use power button to start it again. (No shut down animation, it just go dead)

The problem starts when I stopped the Google Now. Yes I am sure. Last night I stooped it from settings. And since then phone is shutting down by itself.

So should I again start it. But I don't want it.

Any help...??????

Phone shutting off when I turn screen off

So everytime my screen turns off it turns off my phone itself and I have to restart it. It turns my whole phone off.
I am currently running stock rooted with s-off. It just started happening after I restored phone from a previous backup. Everything went great and worked but when I turn off screen it turns off my phone.