Run the fans at 100% and see if it gets worse.

You try clearing CMOS?For my 3200CL14 kit to run 4000CL17 i have it at 1.35V, SA at 1.20V, IO at 1.15V but if you said that the same issue with the Z370 and on the Z390 even at 2133/2666 then there probably something wrong with the kits themselves?

If I keep them on AUTO it will put some insane numbers so I manually put 1.15/1.2.CPU @ 5GHz 1.25-1.26VRAM @ 4GHz CL17 1.35V