If I use FCPX for any length of time, export my project, it often has break up lines appearing throughout the film.
The solution has been to restart or shutdown the mac, but I've not experienced this before in 20 years of editing. I thought maybe it's something to do with the gfx cards but nothing is showing up in diagnostics!?!
I haven't seen that before. What is the codec, and what version of FCPX and macOS are you running? Can you upload a short clip or a frame grab illustrating the problem?
The description sounds like a hardware problem. Apple diagnostics will not find everything. Genius Bar techs at Apple stores have access to much more rigorous diagnostics, including overnight stress tests. Sometimes these tests will find elusive problems.