Did you already apply the Supplementary Update? I was facing a lot of issues also on my rig now suddenly with the i9-7980XE after the 10.13 Supplementary Update.. It seems practically useless... As soon I login into the system I face a reboot just shortly afterwards... If I remain at the login screen, the system stays stable without issues. In contrary, under 10.12.6 still everything works as expected! The system is absolutely stable and has no issues, despite the change to the i9-X7980XE! Could the latter 10.13 Supplementary Update be the source of all evil? What about downgrading to 10.13 Final Release?