We recently bought 2 new Optiplex 7020 computers and both have the same problem. One was returned, but we had to keep the other as the box and related materials for the other had been disposed of. 

The problem is that within a few hours of power up, it will completely crash, with the screen turning into flashing patches of pixels. It is at that point completely unresponsive and has to have the power button pressed for 30 seconds to shut it down. To attempt to resolve it, I have  upgraded it to the latest OS version, installed all patches, updated all drivers, cleared the BIOS cache, run the Microsoft DISM fix, run the sfc /scannow fix. It shows no problems what so ever until it crashes and I haven't identified any helpful errors in the error log. It is only running Microsoft Office, Zoom and Teams. Any ideas on what might cause this? Both computers crashed identically.