I currently have a SLI configuration of 2 EVGA GTX 670 4gb SC+ cards with a 3 monitor setup. Over the past month I've begun to have issues from one of the cards displaying green squares and the display driver crashing. Originally this was only happening during  surround gaming but is now happening without any program running in both surround and non surround mode.
I have tried a couple different things but to no avail, the cards are powered correctly, the SLI bridge is seated properly, and they have good airflow and temperature. Rolling back the Nvidia's drivers had no success either.
I also tried running a single monitor off the "bad" card and it crashed once and then I didn't have any more problems with it until I reinstalled the second card and tried my original setup.
 
Any ideas? Could just be a bad card, but was wondering if anyone had any other idea's? Been working fine for 4+ months.
 

Intel i7-3820 @ 3.60Ghz
ASUS Rampage IV Formula
32Gb Corsair Vengeance Black @1600Mhz
2X EVGA 670 4Gb SC+
Corsair Professional Gold 1200W
Corsair Neutron 128Gb SSD
WD Caviar Blue 1Tb HDD
Corsair Hydro H60 Cooler
Windows 8 Pro x64
 
Hope thats everything important :) My PSU is only a little over 4 months old along with all of my other components.