Some problems while overclocking

[BlueBullyXZ]

Now, with the uncoming Battlefield 1 Beta, I was doing some overclocking with my EVGA 960 on the Valley 1.0. But with whatever voltatge more than stock that is 1,206V this uncome some unstability, like on most parts of the benchmark the clock gets down from 1550Mhz to 1534Mhz also with the voltage, from 1,225V to 1,206V. Why is this happening?

[KE2012]

What is your temps? 

[BlueBullyXZ]

64 to 66 degree at 65% fan speed (about 2200rpm)

[KE2012]

Open Nvidia control center, go to 3d settings and click prefer maximum performance. 

[BlueBullyXZ]

I did this in 3D Settings->Global applications->Power save>Changed optimal by Maximum performance and nothing changed

[BlueBullyXZ]

I come back to say the change that I did was quite unnessesary (I don´t know if this solved the problem). I turn up the voltatge from 18mV to 25mV and now it´s stable. But when I played more with the Core, I went up to 1593Mhz and chashed inmedeatly and then the card wasn´t going above 550Mhz. Is this kind a life save for the GPU?