Sony nxcam hxr nx5u shutts off on its own

Hello,
I have a Sony nxcam hxr NX5U, today when I was shooting a live concert, the camera was keep turnning off. I thought my SD cards have errors which results in shutting the camera off. I changed the cards in fact I tried couple other cards but it was doing the same thing. Later, the phone was on stand by and after couple mins the camera turned off.

Any idea why its happening?

One more thing, my camera is on auto exposure and I can't turn it off, when i am zoom in, the f-stop goes from 1.6 to 4. I can't figure it out how to fix this issue. Any help would be greatly appreciated.

Thanks