Wow, do you have a big fan blowing on them?  I have 5 of those and they are usually only overclocked maybe 75 on core and 325 on mem. As it is they are scalding hot when you touch the heatsink after they've been working for an hour or more.  When the boost OC would start approaching 1275mhz, it would freeze ccminer or start acting strange.

I do remember my overclocks seemed to be much more stable when we used to deal with scrypt or just a few releases of ccminer.  Now it seems the more functions and algos ccminer has, the less reliability of my overclocks.  My rig of EVGA FTW Ti's is pretty much the most stable thing ever on cudaminer, ccminer, and all the other miners I have tried, but it can still "only" do +10 cpu which puts it at something like 1349 boost clock.  I don't know how some of you get those clocks nearing 1400 on the other cards...