I have a Radeon RX 5500 XT and when I use my monitor at anything above 70hz I get these black lines on the bottom of my screen that get progressively worse when I turn it up. my max on the monitor is 165hz and it basically covers half of my screen. I've used DDU, updated drivers, check cables, unplugged pc, restarted, and everything it still doesn't work. It also happened randomly I don't know what caused it though. My frames also drop all of a sudden as well when I load up a game but when I open task manager it runs smoothly again sometimes it still runs slow tho maybe a windows problem not sure. 
I have the same problem with a 144 HZ AOC screen when I start my pc, lines appear at the bottom of the screen and it is becoming more and more difficult.
