Before you start doing major hardware replacement troubleshooting, try preparing another usb key with the free version of unraid, just don't assign any of your current array drives. You can install and run unmenu on the new key without any array drives, and that should be a good test of whether it's the usb on the board, or your key that is causing the issues. When my 1st key died, I saw similar behaviour, where it would boot ok, but after a period of time, the usb drive basically just disappeared from the system.
Unfortunately, the blank monitor symptom makes me think it may be the motherboard, CPU, or RAM, but who knows, you still could get lucky.