Just an update:
Boot the PC with memtest, 16GB Single Channel, with 8GB each DIMM, slotted @ DIMM 1 and 2. No Error. Yay..
Boot PC with another 2x8GB on DIMM 1 and 2, memtest produce 116 errors. :-(.
Reboot, and successfully boot windows with the "faulty" 2x8GB.
Shut down, and slot all 4 DIMM...