Try reversing the polarity of the green/white wire.
my Opus 150watt psu is hooked up to my carputer
but if I use the green/white wire to pins 6/8 (respectively)
the carputer starts up windows then shuts down again
While the setup was being built on the bench I just used a simple momentary switch connect to pins 6/8 on the MB to start and shut down the pc
I think the Opus is sending a constant signal and not a momentary signal which is causing the shutdown
my motherboard is a Intel D845GRG and based on the PDF diagrams on the intel site I am using the correct Power switch pins and not the reset pins