If you're not a computer wiz like me i would not try to experiment with the voltage. And probably then it will still not work properly, because of the different settings between idle and in game. With the right driver it does that automaticly, i believe even with oc guru off, but i'm not sure of that, i like to run it with oc guru on anyway. Did you also download the latest drivers ? ( watch out you get the right ones, there are i think 3 or 4 versions of the SOC, when you select one you get a picture of the model, it should be the excect same as you have ! )
Just do a "clean" install of your new drivers and than run the bios utility, first back up your old bios with the utility and than check if that version is older ( you check it by running " flash "), also with the utility, it will explain itself. When you have to check the version you will see something like GV-N560SO-1GI-900, you can click on that, then open. It will than probably say ; the bios version is the same etc. This means you have got the latest version allready and when it doesn't it will flash to the newest bios. Even i could do it so it can't be really hard. Good luck !