Welcome to the EVGA forums
 
There are a couple of issues to look at when adding a second card for SLI causes instability, particularly a dual GPU card.
 
Your system overclock needs to be reset to defaults for testing.
It "may" need to be re-done, as adding a "second" videocard will quite often change MB/chipset/Mem behavior enough to render a previously known good overclock unstable.... 
 
The other thing to look at is your system RAM and timings.
When running a 64gb RAM load, even the slightest timing or voltage problem can cause serious stability issues
When running 600 series cards, the way in which each GPU core interacts with your system RAM (memory extension) and the fact you have (4) such extensions running with quad SLI could cause system RAM problems. 
*The best way to -quickly- test if the problem might be RAM related, would be to remove all RAM sticks -except- for the first (2) sticks in the primary channel.
You should then test with just that 16gb. If the issues go away at 16gb, then you can focus on loosening timings or voltage adjustments as you (add back in) banks of 16gb (2) sticks at a time and test...
 
*there is also the issue of your current MB BIOS version, and if you are running one of the latest updated BIOS versions that contain the latest GPU compatibility fixes -and- the known PCIEx 2.0 vs PCIEx 3.0 issue... so certainly test with the BIOS set at both PCIEx 2.0 and then again set a PCIEx 3.0
 
I would focus the the first two steps noted and the MB PCIEx BIOS settings, before considering the BIOS flash
*(unless your current mb BIOS version is really out of date, if it -is- then that needs to be flashed)    

I tried all of your suggestions, but none work. the card in slot 3 runs fine with SLI disabled for that card, but can't complete the benchmark tests with SLI enabled.  And, of course, the benchmark tests freeze almost immediately with quad-SLI engaged.  Any thoughts?

ElandauNB

Further information which may help someone solve this problem.

I've been able to run a complete 3D Mark 11 test with the Slot 3 card by fiddling with the monitor output.  Here are the scores:

Overall: 5513 (one third the score of the slot 1 card in SLI)
Graphics: 4945
Physics: 14188
GT1: 29.45
GT2: 24.01
GT3: 30.95
GT4: 12.8
PT: 45.04
CT: 24.33

Help would be appreciated.  

Yeah my GTX 670 single GPU scores way more than that in 3DMark11. Something seriously wrong there. Afraid I don't have any experience with quad-SLI or dual GPU cards though =/

Had some similar issues when adding the second 690.
I went into BIOS, and set the PCIE's to GEN2.
Problem gone.
 
Later I did a bios update, then it was ok to use GEN3.
 

I have updated the BIOS and video drivers, but not the chipset. That's next . I have set the BIOS to Gen2,but it didn't help.

I'm running a single Dell 30" monitor with both cards. Have you tried that? I only can use DVI.

I hope the chipset update works because I am running out of options.

Thanks everyone for the assistance. I'll keep you posted.

Also one more suggestion.....
You have dedicated soundcard? If so, try remove it.
There are some theory's on soundcards may cause some PCIE issues. 

Try any of these settings, might also be a combination of few.

Disable HPET in BIOS
Disable Xtreme Tweaking in BIOS
Adjust BCLK Skew - Trying -1, -2 , +1
Adjust PCIE Clock Skew under CPU Paradise - Trying +10, +20, +30, -10, -20