just today i started having the same problem (up till now everything was fine). Yesterday however i boosted core and memory clocks, hence i believe it's related. I'm lowering now the mentioned settings to +80/+610 in afterburner (effectively a loss of 1Mh per card) and will see what happens. Stay tuned.
i confirm that it's all about OC/UV. After i changed settings and restarted, the problem went away.