I recently bought a used rig. All partds were put apart but I was confident enough I could put it together so went for it!
In the end it's been a pain! Apparently all because of the motherboard.
This is an ASRock H81 BTC Pro with 6 PCIe slots where I plug 6 SAPPHIRE RX480 Nitro+ 8GB. Power supply delivers up to 1200W.
First of all I tried to boot the PC with only the CPU 5celeron @2.8GHz), RAM (4G DDR3) & motherboard and a VGA display plugged in. Things started pretty badly as I could see the CPU fan spinning for a few seconds, then stoping, then spinning again for a few sec, then stopping, etc...
I had to reset the BIOS in order to finally get the system booting properly and install my OS.
Installation went fine and I also installed the AMD blockchain drivers 17.50 for Linux.
Everything became really dodgy when I started to plug more than one GPU to the system. After pluging the 2nd GPU A message appeared on screen prior to POST asking me to plug to the motherboard 2 4pin molex connectors (this card has 2 slots to plug such connectors). The manual stated that it was only required when the system had 3 GPU but for me the message appears starting at 2 GPUs....
So I plugged those connectors but the message remained (Itried several connectors) and I finally had to ignore it to keep going.
I then added the GPU one by one but usually after adding the 3rd ones the system refused to boot and I had the same behaviour as the first time I boot the mobo... cpu fans (and GPUS) spinning and stopping. Like the system is booting in loop.
I once managed to have the system to boot with the 6 GPUs connected and detected by the OS. I started mining (pretty good hashrate I think, 800H/s per GPU no monero) and after 3 minutes or so the system froze and I couldn't reboot until reset the BIOS again.
In the end I had to use another motherboard I had (supermicro server X9DR3-F), which is really not optimized for mining but which is doing the job reliably (cosumming a total of 915W mining with the 6 GPU)

I wanted to know if anyone ever had similar issues and maybe knows a magical trick to make that H81 BTC happy again, or is it just a lost cause and I should trash it?

Regards