Well, first it would be good to find out what the temperatures of your cards are.
It may just as well be an overheating issue. Have you tried installing Speedfan or some other temperature monitoring tool?
Also, are your GPU's overclocked?
Overclocking increases temperature and increases the amount of power the GPU needs, wich in turn heats up the voltage regulators and could cause instability.
Also, a powersupply that cannot keep up could also create this kind of problem.
Under load, gpu1 59c , gpu2 70c.
No overclocking, all stock speeds. trying to figure out the best setting for the cards to last a while.
under MSI AB, GPU1 usage % 89, GPU2 usage % 99.
I'm not going to lie, I'm a cgminer newb But to me it seems as if gpu2 crashes after so long.
Any help is appreciated