You may be using faulty metrics. You said your supply was 850 watts, and you're wondering why things are failing because you're only drawing 640 watts.. What you're failing to take into account is the 850 watt rating of the power supply is for ALL rails combined. The actual power rating for each rail separately is always much lower, there's a very good chance that you're over taxing one of the 12V rails under this false asumption that because it's an 850 watt supply that it should be able to do 850 watts on the two +12V rails, this is not the case. You'd have to measure the amperage on each separate rail and compare that to the listed specs of the supply (they do list the max current for each separate supply along with the total wattage limit)
Suffice to say you may be using a power supply that doesn't meet the system needs, not the PSU itself being faulty. It's very possibly a system design fault not an actual device fault. Toss a 1500 watt supply in there and see what happens. NEVER go cheap on your power supply, and ALWAYS over engineer or you run into these kinds of problems.