I'm building a new mining rig but can't seem to get past this problem.  Each GPU can run with gpu-threads 2, but if both are set it will only mine for a few seconds before the video signal fails and the GPU fans spin down to idle... the system then requires a reboot.  I've tried various Catalyst drivers, litecoin_bamt installs, powered risers, GPUs mounted directly on the MB, even flashed their BIOS to run a lower voltages.  Together, they pull under 500w before it crashes... no visible power spikes or other oddities.  I'm running out of ideas, so I'm willing to try just about anything.  In fact, even if I've tried your suggestion in the past, I will try it again and post hard data where applicable to best document this problem (and solution hopefully).

Here are some specifics on my hardware:

I used this guide: http://www.coinminingrigs.com/how-to-build-a-6-gpu-mining-rig/
MB: MSI Z77A-GD65
CPU: Celeron G1620
RAM: Kingston Hyper X Blu 4 GB
PSUs: 2 x EVGA SuperNOVA NEX750G
GPUs: 4 x GIGABYTE GV-R928XOC-3GD rev2 r280x
I've posted about this problem in another related thread, but the replies get lost and so does the momentum for finding a fix.  I'm hoping I will be able to update this thread with the large bold SOLVED before the weekend is over

Related thread I posted in... (Most of the suggestions centered around power issues):
Original post: https://litecointalk.org/index.php?topic=2924.msg110819#msg110819
Recent post: https://litecointalk.org/index.php?topic=2924.msg134596#msg134596

I have also posted this on the litecointalk.org forum and will update both if a solution is found:
litecointalk post: https://litecointalk.org/index.php?topic=16519.0