A buddy of mine had this issue with the same card on his Razer Pro. Funny thing is he was golden for almost two weeks before the GPU finally fried. As Baby said, laptops aren't meant to be overclocked much if at all as they generally have terrible ventilation issues and poor cooling systems. As far as I know the 780M is soldered in, which means you're pretty much screwed. I'd get a whole new rig as this point as if you're overclocking you're clearly looking for a performance boost, and next time you do it make sure you take baby steps or you'll relive this nightmare all over again.