Z97 Extreme 6 (BIOS P2.70), i7-4790T CPU, 4 x 8GB Kingston memory (KVR16N11K2/16), Samsung 850 Evo 500GB ssd, Corsair RM450 PSU, Fractal Core 2500 case, Liteon ODD, Generic/Asus nVidia GeForce GTX 750Ti-2GB GPU, Mellanox ConnectX-2 10Gbps NIC.

I assembled this system back in Nov 2014 (10Gbps NIC + 850 were later upgrades), and it has been serving me faithfully until today.
After first moving it from my study to the living room to watch MotoGP streaming on my TV (where it worked fine) and then back to the study, the system now fails to boot.

With BIOS A selected, it will halt on code 55 (Memory detection), but it will go past this when BIOS B is selected to the splash screen or all the way to the UEFI BIOS setup if no SATA devices are attached. With SATA devices attached, the system will stop with codes like 62/64 (chipset init failure), 9x (PCI-E problem) or Ax (IDE/SATA device problem). With no SATA devices, it will go into UEFI BIOS.

I've re-seated all cables, memory, and CPU. I've removed all PCI-E and SATA devices as well.

Any ideas? Anyone seen something similar? - my guess is a failing/dead MOBO, but would like to hear ideas/experience before getting a Z97 Extreme4 board (Extreme6 appears to be EOL).