I recently bought a 2009 Mac Pro locally that is flashed to 5,1, though the boot rom is 138. It’s also on the latest Mojave update.

It’s been having annoying graphical artifacts on the screen after it’s been on for a bit. At first I thought it was the cpu overheating or something because it would usually happen after the fans would go up by converting a video or playing a game. Finally realized it’s because of the gtx 660 ti that’s in it after some google searches. (Annoying because the seller said it was Metal capable...)

So I’m thinking I have 2 options. Either somehow downgrade to Sierra and keep the graphics card or spend roughly $200 to get an rx 580 with the power adapter cord. I looked up how to downgrade the os and it seemed simple enough but I couldn’t make a bootable usb drive with the downloaded Sierra dmg file from Apple, saying it was missing a certain install dmg file inside it. If I can’t do it that way can I wipe the hard drive completely and select Sierra as an option for a fresh install after going into recovery mode?

The 660 ti seems plenty powerful and I’d rather save $ on upgrading to an ssd than getting another graphics card.