What tests did you have in mind?
For me I can get any of my not 100% stable cards to crash in World of Tanks with enhanced graphics on, 16x Anisotropic Filtering and 4x MSAA with 4.5 GHz 3570k @ 1080p.
This will start artifacting within 3 hours if the card is not 100% stable.
Another good test is MechWarrior Online with most of the image dehancing shaders off, since its DirectX-9 with Cryengine 3, basically the perfect storm of utilization. As long as you make sure you are not shader or CPU limited, it will crash or artifact at 90% of the rate of the above World of Tanks test within 2 hours play.
I'm thinking this is probably something to do with 79xx's inferior ROPs.
It becomes far easier to see this happening if you put your RAM at max stable clocks, since the memory bandwidth + inferior ROP is what's limiting the card into not crashing as often as it should.
The memory on my cards is never itself the problem since it doesn't throw memory errors until 1800+ mhz.
I'm pretty sure it's just the ROPs.
You could also get your cards clocked higher stably on older drivers than newer drivers since the older drivers had just plain lower actual utilization.