You can log the voltages with fewprograms and check after

It is cwt psu

Also what is mobo and bios version

Z390 strix and latest bios on site

Stock cables or fancy

Fancy of course, they make your CPU faster.

Fancy cables are known to fail, especially custom made ones. Try stock cables.

Coincidental timing, my custom sleeve cables are causing my 12V rail to sag and thereby causing my PC to spontaneously reboot under load.

It was the connector on the PSU side of the MB cable, fixed now but took me a few mins to find.

I went through 2 diff custom 24pin mobo cables, before just getting a small extention to make it look purdy. Both had failed on me, had even replaced the psu because it failed paperclip test. Was just the cable tho

Using a spare supply right now and no issues so far. Time will tell