If the case is open, then there is no airflow. There is just air around the parts, but surely no flow. And the 7990 is no cake to handle because of its temp and power issues and you got two of them. But still that crash you describe is a hefty one. That's definitly a hardware problem and not related to drivers or slt.
You ruled out the PSU so that should be fine. Did you monitor temps and Watts while running oclhashcat? Have you tested the memory properly? Did you try some other extensive benchmark i.e. Luxmark? Anything overclocked? Newest BIOS? Are the 7990s used ones so they might be damaged already (maybe by some gruel Bitcoin miners)? Did you try each card on their own to see if the system is fine then?
You ruled out the PSU so that should be fine. Did you monitor temps and Watts while running oclhashcat? Have you tested the memory properly? Did you try some other extensive benchmark i.e. Luxmark? Anything overclocked? Newest BIOS? Are the 7990s used ones so they might be damaged already (maybe by some gruel Bitcoin miners)? Did you try each card on their own to see if the system is fine then?