Z620 Workstation - System Unexpectedly Hangs, Reboots, or Shuts Off

My Z620 workstation may work for hours without any issues but occasionally it restarts or shut off on its own.
It's unpredictable when it may happen. Sometimes it takes few hours until it freaks out and other day it works just fine. I have latest firmware 3.88 and Windows 10 OS. I have installed fresh OS instalation and it doesnt make any difference. All drivers are up to date and it's standard Z620 model. I don't know how to claim it to HP since it's very difficult to fault trace... I still have one month warranty left but beacuse of intermittent nature of this problem I may have problem to claim it. Any help appreciated. I'm located in Belgium.