Strange overclocking problem

[PurpleNinja63]

OK so i got a new GPU (EVGA SuperSC 4GB 960) and i was overclocking (I've never done it, didn't overclock my 670 cause it was my first card, my baby) and well I've run into a wiered problem, whenever the clocks get over 1500mhz for a little while, the clockspeed will drop and stay at 539 until a restart. Why?/can i stop it doing that.
Thank you in advance

 

http://gyazo.com/624487c42cfb6301dc097c3cd9105e9c - No overclocking

 

http://gyazo.com/cc85cb393c66affc8d70cc6288ce06c0 - Overclocked but not enough to get over 1500mhz (no problems)

 

http://gyazo.com/83d3f6b74e0d50e83fd3ae0826eb6fed - Overclocked to 1540 (as you can see in afterburner i got above the 1500 for a while on a lower clock which got to 1520, so i put it back up to 1540 which is where i had previously put it straight away because of how low my temps were, so I don't know if its i didn't have it at 1520 long enough or it doesn't have the problem until it hits 1540, only reason i think its because of the over 1500 is because that is afterburner limit on reading.)

 

http://gyazo.com/99e738b55c265908d2345010136b4f5a - Then it does this after a little bit at 1540, and will not get higher than 539 until I restart PC

 

 

[Godlygamer23]

Can you unlock voltage control?

[PurpleNinja63]

Can you unlock voltage control?

maybe. how do i do that/ how would that help though it's only drawing %92 of its maximum power consumption