You must be overloading one of your rails to the brink. Have you tried moving the equipment around onto other rails.?
There's only so much I can do. My PSU is the Be Quiet! e9 700w which is internally similar to FSP Aurum series(the PSU is also manufactured by FSP). 5970s are on the pcie power cables and my 5870 is on the CPU 8 pin. the last 5870 is powered by another PSU. Just starting cgminer so that both cores of the 5970 start will trip the OCP on my PSU. The mobo is also powered by the e9 however the CPU power comes from the other PSU.
This is a bit funny way to do things I guess but the 5970s were picky about power not going up simultaneously on the mobo power and the 6pin.
As far as I can tell I shouldn't be overloading any of the rails. It could be that some PSUs aren't so good with high transient loads.