Z5 restart issues

[B2578]

So I just got a Z5 and the issue I'm having is that it will seem to be working fine then it will all of a sudden turn itself off or restart and sometimes it will turn off and come up with the battery sign as if the battery is dead but the phone still has charge, it's becoming annoying as I can never tell how long I'm going to get before it does it and it makes me not want to make calls incase it happens during one, any advice or solutions would be great thanks