Maybe you should see some detailed specs on the rx series cards... some non reference 580 cards can see as high as 220-240 watts of total draw. No less than 150 at the pcie power plug and another 50 at the slot/riser. This obviously assumes you run at full boogie. I feel your 1200W psu leaves no room for the mb, cpu and mistakes.
Do not underestimate ethereum dual mining. There are significant surges during mining, especially when intensity values are at the ragged ege or just plain too high. The cards will tdp limit, pull clock, release limit, rebound clock to setpoint and repeat. Under severe situations they will drop usage to 0% and back to 100%. My experience is that the tdp hammering is noticeable and cause current spikes on the psu's.
If you do not undervolt + reduce power limits severly you will have problems IMHO. Such a severe underpower will cripple mining hash. Many use 1200's for 6...I am stating facts and experience. I had 3 rx series on a good 750w psu surge up/down enough to trip the overcurrent protection and shutdown the whole system due to my stupidity and shit dual claymore settings. In the process the hot gpu's were sizzling and popping like bacon as the thermal paste and pads started boiling/melting. If I wasn't there to save the day all cards would have been lost.
Reference power is 150+50=200, non ref = up to 240. Suppose you pull down the power 25%. Now your at 150 or 180. X6=900 or 1080. Add at least 50 for the cpu, another 50 for the mb...now your at 1000 or 1180. Factor in surges and at least another 10% for safety margin and you are well shy of the requirements. Yes I know there is 1200w on the 12v rail alone, save the lecture... I am simply doing simple math to stress my point.
Don't take my word for it. Read the power draw from these test jockeys rig slap together using a shit cpu that cant handle the job and using risers with sata connections on the power hungry rx series. All that is missing is the magic smoke and insurance claim. 1020w at the wall with dual 750's and moderate power savings applied. I do like their hash rate though...
http://1stminingrig.com/amd-sapphire-nitro-rx-580-8gb-limited-edition-mining-rig-tutorial-190-mhs/Ps...power limit reduction doesn't equal stability, only power consumption and indirectly temperature/lifespan. I guess one could argue temperature and instability...but we're splitting hairs here. 70% tdp shouldn't be a requirement for a stable system. I can run all 1070's at 112% all day long if I wish...it's stupid but totally stable there. Amd is no different. I frequently go from economy mode to kill mode when I see huge market surges or new coin offerings at low difficulty...
