Well it is certainly not a fix, but I turned off secure boot in the recovery menu, siri, sleep, mostly because I don't need them on my imac pro, but that certainly isn't a fix. But it is a better fix than the guy on the Apple Support Forums who on his imac pro support case was told to buy an external video card to fix his problems (quoted below). ha.. well at least mine only told me to wipe and start over.
iMac Pro 8 core Radeon Pro Vega 64. I have the same problem. I had contact with the support all year. The diagnostic tool ran several times on my system. Several clean installations of 10.13 and now Mojave didn't help. Support advised me to buy an external graphics card. I am incredibly disappointed.