My replacement board has just arrived. It doesn't work well either. At first I almost thought this board was ok, when it gave me a fair score on Heaven 4.0 with a single GTX 780 ACX SC on slot 3. However, after I restarted and benchmarked the computer for the 7th time, things began to go wrong: more and more frequently, when the computer restarted, it gave no video signal output with the on board VGA_LED in red. Though without video signal, it could still load windows 8 and perform normal shutdown when I pressed the power button. On the other hand, in dual card configuration(slot 1 and slot 3), the card on slot 1 worked well and always gave me signal output, but half the time after restart the second card on slot 3 would go missing on the Nvidia Control Panel.
Later, I spent 2 hours and tested each graphic card individually on slot 1, slot 3, and slot 4 for 10 times. It turned out that things would only go wrong on slot 3.
I am done with experimenting for the moment, and using SLI in slot 1 and slot 4 to get by. Hope some smart guy would stumble upon this thread and help me figure it out.