B250 Mining Expert rig with RTX3080s crashing and freezing

Hi! I have seen some posts about rigs with the Asus B250 Mining Expert, but none seem to be touching the issues I am having. Can anyone suggest possible solutions / troubleshooting? I am posting my config and issues below.

RIG:
Asus B250 Mining Expert + i3 6100T + 32GB RAM
6x RTX3080
BeQuiet! DarkPower Platinum 1200W
Dell Server PSU 1400W
Samsung 980 Evo SSD
Blue risers, all powered directly with 6pin PCIE connectors
Latest HiveOS and GPU drivers, latest BIOS revision, PCIE set to Gen2
Display is hooked to the Intel iGPU

The rig starts (most of the time) with GPUs plugged into consecutive PCIE slots on the MOBO. I am not using the x16 slot, only the x1s. After booting, it immediately starts mining, but when this happens, I am getting artiffacts on my display - it starts flashing and I can see some vertical lines popping. It stabilizes then and goes on with mining, but whenever I want to send a command to it or stop the miner (locally or remotely from the Hive panel), it dies and requires a reeboot. After a crash like this, most of the time, the system will not post - it displays a non-blinking underscore sign before even going to BIOS. I need to turn the power off and wait some time before it recovers.

Troubleshooting done so far:

  • I have tried with different GPU connection layouts as I’ve heard the B250ME is quirky - same story.
  • I have tried to plug each GPU one by one to check its limits. It worked somehow stable with 3 GPUs and crashed the same way immediately after I added a 4th one, regardless of the card plugged in.
  • Checked the logs, seen two worrying messages there: one was a xorg issue with screen init problem and the other one was a CPU lockup. They are not consistent though.
  • Changed the CPU to some compatible Intel Pentium G series and the issues persisted.
  • Checked the PSUs, since I have a spare 2kW one as backup, but it did not fix my problems.
  • Did a memtest - passed without issues. Checked with only a single 16GB RAM stick - same problem.

What could be a source of the system being unresponsive? Is it the GPU connection layout? Could it be a broken MOBO? In case I am powering my risers directly, do I need to also plug in the AUX_PWR molex connectors on the motherboard? I am running out of ideas here. Any help will be much appreciated!