One of my users has an aluminum/glass iMac (8,1) with Radeon 2400 video. For a couple weeks now the display will eventually dissolve in static; the wallpaper largely disappears and so do some of the icons, but when you move the mouse pointer around the screen returns to normal under where the pointer goes, likewise if you click-and-drag on the desktop to make a box.
He took the Mac to a repair depot, where they replaced the motherboard, video card, inverter, and something else. The tech asked me to do an archive-and-install of 10.5 to flush out the old video driver, in case that was corrupted. I did so, didn't have any problems for a couple days while it was getting set up and updated. After the user had it for about that long, the static came back. On a hunch I ran a program to check the temperature sensors, but nothing was unreasonably hot; 50 Celsius was the hottest for any of them.
 Non-moving static; my user describes it as "plaid".
i7-8700K|Asus Z-370 Pro|32GB DDR4|Asus Radeon RX-580|Samsung 960 EVO 1TB|1988 Model M||Logitech MX 518 & F310|Samsung C24FG70|Dell 2209WA|ATH-M50x