IMO, there are four possibilities:
1. Defective hardware. Since you have already eliminated the PSU, your only realistic option would be to return the unit and get a new one. If we can eliminate the other possibilities, that would be IMO the next step.
2. Corrupted flash. Although you have already done a factory reset, you might try reloading the firmware, then resetting again.
3. Incompatibility with router or switch to which it's connected.
4. Firmware bug, triggered by unusual traffic on your LAN or from your provider.
Can you connect the unit through your PC? For example, PC connects to router via Wi-Fi, Internet Connection Sharing on PC of Wi-Fi interface to Ethernet interface, OBi connected to PC Ethernet port (via crossover cable if needed).
This would do three things:
1. It might stop failing, because it would see a different physical Ethernet interface.
2. It might stop failing, because it would be "shielded" from most broadcast traffic on your LAN.
3. If it still fails, you could easily capture traffic with Wireshark, which may show what kills it, or if any packets are still being sent or received, when in the crashed state.