@alrazule - I ended up fixing it by baking the GPU and that fixed it for now. A longer term fix will be to replace the GPU but as it’s already 10 years old I’m not sure it’s worth the investment. I did follow along a thread about replacing with a modern GPU so that it can run Catalina but again I’m not sure that’s worth the investment. If you’re interested in this route the thread has a wealth of information:
https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/
Good luck with your iMac!