I stressed everything except the local disks. And to be sure I even stressed just the CPU for around 40 minutes. But it was stable.
OK so yesterday I tried playing the NFS mw (older one I think 2005 one) but the system didn't reboot. That's surprising. I think the recent windows 10 install did the job for me... I'm not sure the problem has been solved completely so today I'll check with some other game.. thanks