it happens ONLY when the core clock changes to 960Mhz or 1000.. etc from 300Mhz.
If there was a way to keep 960Mhz as default core clock, then i think it could work fine.
Have you checked to see if the speed setting on the PCI-e slots has changed? (ie are they set to 2x, 4x)
Also, does it matter what card you lastly start mining with that causes the crash? Have you tried swapping the cards around to see if you can isolate the issue to a vid card or a PCI-e slot?