macOS Sierra Patcher. Download and instructions available at http://dosdude1.com/sierrapatch.html …pic.twitter.com/85Tis42Z1r
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Yep, that's why... It's an incompatibility with newer Radeon graphics cards on the Mac Pro 3,1. Unfortunately, the only way to actually fix this is to use an nVidia graphics card, otherwise you can just not let the machine go to sleep.
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