I have an Imac mid 2011 and it started on Mac OS Mojave with an AMD Radeon 7500 graphics card. I used Dosdude to upgrade to Catalina recently to enable more capabilities for my Imac. Then I changed my graphics card to an NVIDIA GeForce GTX 675MX with 4 GB of RAM. Since then, I’ve been experiencing weird horizontal lines on the right side of the screen that blip quickly in and out of existence. Sometimes they’re white, sometimes black, and recently I’ve even noticed them turning red. And now I’ve noticed that also it doesn’t like going to sleep for at least 10-15 minutes because, after that time has elapsed, it seems to crash when I turn it back on. It starts off as a black screen showing my mouse, but it doesn’t let me MOVE my mouse. The lines are, i’m pretty sure, radio-static lines. But I have no idea what caused the going-to-sleep-then-crashing problem, but I think it’s to do with the NVIDIA graphics card.