what are your target clocks? i have zotac gtx960 dual fan edition that is currently oc but not overvolted. +120mhz to the core, +150mhz on the mems.
I have gtx 960 msi stock with coreclock +170 and memory clock +530 . And the maximum I can reach was coreclock +180 and memory clock +550 . It's not safe with second overclock because is the maximum of maximum ... These settings without overvoltage. And with voltage at maximum (+100mV) coreclock +190 and memory clock 580 .
P.S. Sorry for my bad English .