This happens with all games. Both cards on their own run all games flawlessly. When they are in SLI, the framerate stutters and sometimes the whole screen will lock up for a second or 2. Just recently while playing Starcraft 2, the whole screen went black for about 7 seconds and then came back, with my fps at about half what it usually is.
Both cards are great when they are running on their own. Only SLI is causing this.
GPU-Z shows the EVGA card running at PCI-e 3.0 consistently. the Asus card dips to 1.1 on a regular basis, in or out of games.
I doubt its the PSU. its a 1200w Corsair and shouldn't have any trouble with 2 670s.
Drivers are the latest from Nvidia for the 670. I'm not using the beta drivers.
I recently updated my motherboard's BIOS to see if that would work, and it didnt.
What PCIe slots are you using? You should be using the blue slots to give lots of space between cards, and to insure you are in x16/x16. You may also want to try 1 card in both slots on their own, to insure the PCIe Slot is working properly.
I am using the 2 blue slots on the motherboard. I just switched the cards to see if that will make a difference. GPU-Z now reports the EVGA card, now in the bottom port, is fluctuating between PCI-e 2.0 and 1.1.
The Asus card started life in the top blue slot, and i moved it down to the bottom one after I anticipated getting another 670 to go with it.
The Asus card ran great in both ports before i got the EVGA 670, so that is one problem ruled out.