One of my hashboards crashed at 600mhz running at 77C after about 8 hours. Hashboard came back after a restart. Moved both machines back down to 550mhz.
77C is NOT stable.
Now running 65/64/71, 62/64/65 at 550mhz with 95% fan. Stable so far.
Yeah I pulled back down to 650 after my crash at 700. But I am running 650 and now only 69C on hottest board.