What is your power consumption at the wall? A Rx 580 can pull up to and over 200 watts depending on the model and with 6 of them you have already maxed out your 1200 watt PSU not even counting the other components installed. It's certainly possible that your motherboard may be faulty, but it's a definite that you need either a second PSU or a bigger PSU.  Even if you downclock extensively to lower the power draw you need to be able to mine with your system at 100% stock for at least 24 hours straight to know if it's stable enough to even try overclocking or undervolting etc...