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...
underclock Gpu clock at 1135
default memory clock at 2000
core voltage tried default and 0,91 volt
at wall when i manage to work all together for a little is about 800-900 watts. I don't think is the psu. Cause i have the same instability even sometime with 2 cards! Before i had 2 psus 750watt bronze coolermaster but i put the platinum corsair just to be sure.