Hi ,
The issue could actually be the drivers for your graphics card. The most recent drivers that were released for Witcher 3 for the 600-900 series of GTX cards has been having widespread problems of causing TDRs (Timeout Detection & Recovery) which means that the card itself is becoming unresponsive and tries to recover. If it isn’t able to recover in time, the computer can sometimes restart itself. Can you check what graphics driver you’re using at the moment? This can be found by opening your Nvidia Control Panel. If you’re on a version past 350.12, please try reverting to that driver by downloading it from the following link:
When installing, please be use to use ‘Custom install’ and check ‘Clean Installation’ so that it doesn’t have any issues reverting. After doing this, please restart your computer and see if you’re still experiencing the issues.
Hope this helps!