I have the same issue with my Gigabyte 270X OC 4GB . I remember the first day that Ive tried to OC it. Indeed its a little default OC but you can turn it much more in the Gigabyte OC GURU software.
I could increase to the top that the software allows and play franckly fine and nice. After a while, I dont know if the issue is to reinstall all windows again, I cant turn it the half I used to.
I dont know if send it back to the manufacturer is the solution. I just know I COULD OC it to the software limits and NOW i cant, like magic. And isnt about temps.