Mac Pro Freeze & Desktop Reset to Default

Hello,

I have a Mac Pro from late 2013 running High Sierra 10.13.3. I am having a problem where the video display will freeze and I am forced to hard reset my Mac by holding the power button down. If I happen to be on a VoIP call my call continues fine (I can hear them and they can hear me) however the mouse won't move or if it does move. When I am forced to hard reset my Mac Pro it always reverts one or more desktops to the default autumn hillside by a river whereas I prefer a "solid color" black setting. I've surfed the forums and cannot seem to find the root cause.

Any help you can offer would be greatly appreciated.

MDBik