Graphic glitches on 2007/2008 iMac running core patcher (Big Sur / Monterey)

Hello everyone, Seems like I also need some help. I tried running Big Sur / Monterey on a 2007 iMac (upgraded CPU) and a 2008 iMac. The installation works fine - so does the first few minutes. Then…after a while I get graphical glitches (artifacts) and sometimes lines across the screen that sometimes disappear when I open another window or restart the Mac. At first I thought of a GPU malfunction. However it’s quite unlikely that I would get the same problem on both Macs. An older patch for Catalina was working without any issues. Any suggestions? Any idea / help is appreciated… Thanks 🙏