2x HD 7950's - Eth DAG file Creation Shuts down System

Hoping some one can point me in the right direction

System Specs
PSU: Gigabyte Superb 720W (GE-P610A-C2)
GPU's - 2x HD7950 Saphire Vapor X Cards
basic Intel Mobo with E8400 Processor
4 GB Ram
Windows 7 x64 Pro
2 5600 Western Digital HDD's

So, im new to multi GPU mining,

Whenever i run ETHminer it warms up and starts the DAG file Creation on GPU 0 and GPU 1, when it hits about 96% the system shuts down.

Facts:
- One of the HD7950's is in a Riser card, and powered, and plugged into the proceding PCI-E 1x slot,
- Each HD7950 is on its own 12volt rail from the PSU so the load is spread.
- While troubleshooting: If i run only one card, each card performs flawlessly, starts ethminer, creates the DAG and starts mining
- Ive tried an alternative miner aswell a Duel ETH/DCR miner and it has the same result with multi GPU
- Ive Had the System Multi-GPU mining LBRY credits over night without any problems, each card had a 85-95% load on it over night, and it was just fine.
(however i did notice CG-miner said one card said "dead" when i came back from work while mining that LBRY, upon closing SG-miner and restarting it, both cards came up hashing again without issues. i doubt this related to my system shutdown during ETH Dag Creation)


Im sure i have adaquate power for just those two cards. HDD's and others

Any ideas? i would think its power, but its only 2 cards...