I hope you are figuring your power availability per rig @ 120-160% expected draw? If I ran a 4 card machine I would be for sure running a 1600w power supply.....
I wonder if you are drawing too much +12V off the same rail that supplies the processor and are causing this all to happen.
I have seen many strange configurations once opening up power supplies and seeing what is tapping which available rail. Many PC power supplies have 3-4 independent +12V power supply circuits at roughly 50A each... give or take.... As far as knowing how they are distributed...... you have to open the power supply often to know the real truth...
I dont really agree with the above (per-se):
1) I agree with giving about 20% headroom, but a 1600W PSU for 4 cards isnt necesary (unless they are 290/390 cards or some other varient that would be drawing 300w/card). the rx480 or gtx1070 you could run 6 cards with a 1200-1300W PSU just fine.
2) most quality power supplies have a single 12V rail, and the ones wit multiple rails normally are more like 2-3 rails at 30A each (3x30Ax12V=1080W) Your suggested 3x50Ax12V PSU would be a 1800W+ beast
3) you DONT need to (or want to) open up your PSU and start poking around. youll void the warranty, risk damage, and waste your time. any half-decent power suply will have the power rating and rail ratings marked on it and also on its packaging. If not, use google.
pretty much any PSU that is gold-rated and costs >$100 should be a single 12V rail thats rated at about 95% of the actual PSU specification.
for example, the corsair ax1200 has 1202W on a single 12V rail:
http://www.corsair.com/en/professional-series-gold-ax1200-80-plus-gold-certified-fully-modular-power-supply (click on the technical specs tab)
I know its a late response but.. that psu is setup to draw the mb, 1x graphics,and all the risers off of one rail A1..and part of another graphics card too... very bad design.
