K850 Screen Goes Black And Then Vibrates And Restarts???

[h4$_426]

I have a K850 and evrything works fine, camera perfect but very frequently the screen sort of fades to black and then it vibrates and then it restarts:(. This is very annoying as it happens alot. I have flashed it with a2uploader and completes perfectly but problem still remains. Is this a case of BROD or is it something else??? And how do I fix it? I would love to have it fixed because phone is brand new and has screen protector on it
Please help!:)

Thanks