That’s what I was afraid of.
Now what to do to narrow it down between the PSU, CPU or MOBO?
On the above set up I probed some unused connectors:
On the molex connector (R, B, B Y):
Red = 4.98V
Yellow = 12.04V
On PCI-E connector (B, B, B, Y/B, Y, Y/B)
Yellow/Blue = 12.07V
Yellow = 12.06V...