I'm at a loss to explain it. It's rock stable in everything, but crashes out of 3D apps at 4.3GHz+. I have tried 2 different video cards and its the same. The cards are fine they work perfectly at lower cpu clocks.
I had to set the NB GTL Ref to -70mv or it would crash out in 3D at 4.0GHz, but now with that set up, it does 4.23GHz fine. Dropping to 1:1 ram at a little over 900MHz does nothing, changing NB GTL Ref just makes it worse, changing vNB makes no difference, changing PCI-E freq does nothing, changing clock skews/ram skews seems to help a little but only delays the inevitable. I even changed PSU, and flashed to the latest BIOS, but the same thing happens. I know this board well and I cannot 'adjust out' this issue.
To get over 500FSB on a 9550 you will need to give it a lot of vcore. Depends on the stepping, if its an E0 it will do it more easily but regardless its not going to do it at a 'normal' vcore, and it's almost certain it will not be anywhere near stable. DFI P45's are not very good overclocking quads from what I've read but give one a go if you think its your board.
Well at least finally you got a decent board. Such a shame Asus dropped the ball on the QC on these mobos, but as a result in the meantime you found a way better mobo for less money, so that's something. Good luck with the sale.
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