I have a rtx2080ti and at times, the game just will lose the rendering device. My drivers are up to date, latest being 2/27/2020. After the device is lost, i have to close the blizzard dashboard and reopen. Then when I log in, i only get about 12 fps.
Perhaps your GPU is not recovering from the crash and you’re using an integrated card at that point? The shutdowns of the GPU are triggered by Windows, not the game client, when the GPU doesn’t respond quickly. You could remove your drivers with Display Driver Uninstaller and then try an older version that was stable.