I have the same issue with my iMac when rendering or working using Final Cut Pro X. It happened twice already. I didn’t have this problem before updating to macOS Mojave 10.14.6. This is a bummer. iMac (Retina 5K, 27-inch, Late 2015) 3.3 GHz Intel Core i5 AMD Radeon R9 M395 2 GB I have already reported this problem to their e-mail feedback system and I haven’t got any reply yet. My suspect is this is a software problem (metal & gpu compatibility) as it behaves this way only when using certain apps especially those that use the GPU heavily or those that rely a lot on RAM. IDK