Asking in case anyone else has experienced this and knows how to fix it or what I should do to troubleshoot it? I get these kinds of artifacts (not my video but looks the same) and graphics corruption, where the whole desktop image "explodes" into distorted graphics and after a while it often freezes completely and stops refreshing the screen at all... I have a Mac Pro 4.1 with nVidia GT120. The card is probably 8-10 years old so it may be the graphics card. Other sources have said that it may also be the RAM ("bad ram causing the desktop to draw corrupt graphics"). I have no idea... It happens randomly after startup, especially if I do graphics-intensive things such as browse the web. Also, every time I start up, the login screen only renders the login-background on the top half of the screen; the bottom half is black and just has the shut down/log off buttons but no background image. Almost as if the GPU RAM isn't able to store the whole background image. Has anyone encountered this and knows anything more about it? I have tested 3 OS versions and all have the problem: 10.11 El Capitan, 10.12.6 Sierra and 10.13.4 High Sierra.