You could try to rise a little more the memory clock, if you need rise voltage, do it in little steps since that would add more heat to your GPU and rig.
Please let us know all the components of your rig. Keep rising the clocks until you get good temps and stable clocks, don't worry about "killing" the GPU since with software is almost impossible to do that.
I have an i5 3470 , gigabyte z77-drh ,8 gb ripjaws ram , the highest gpu temperature i get while gaming is 59 C. i also have 2 case fans and am getting 2 more for the top and side panel .
my default core clock is 967MHz and i have it oc to 1052MHz and the memory clock was 1350MHZ and now it is 1385 MHZ. i did not touch the core voltage, should i increase it to get better performance ?
You could try to rise a little more the memory clock, if you need rise voltage, do it in little steps since that would add more heat to your GPU and rig.