You guys are awesome!
fanatic26, you spoted the right thing on the very first post. Thank you so much!
I changed the RAM module for a another one and I could plug all the GPU at once,, while with the previous one it was a matter of adding them one by one and crossing the fingers at every reboot.
It's now running mining ETH on 5 GPUs for more than one hour quietly.
I still have some questions and strange behaviours but I'll open a new thread to tackle them.

Thank you again.

@Fgomes87: ASRock released yesterday an 1.30Beta version of the BIOS. I shall give it a try