Radeon 580 glitching - had to turn on internal graphics

So, I had a fully functional i7 4790k / gigabyte z97mx gaming 5 / gtx 980 ti hackintosh working on high sierra. Then I had to update some programs but they were available only for mojave. So I swapped the gtx 980 ti for a radeon 580 and installed mojave. Everything was running smooth up until yesterday when ui started glitching and videos wouldn't play back, quicktime player and finder preview were full of artifacts.

So what I did was turn on the internal graphics in bios, but kept monitors connected to the gpu and it seems the problem went away. But now I'm afraid the cpu is doing all the work, even though monitors are connected to the gpu and it's still recognized in about my mac.

Anyone been through a similar issue?

Thanks