did you try underclocking your GPU and vRAM? if you have bad chips somewhere in your GPU, underclocking from your factory frequencies does expose them, test that first before attempting the troubleshooting by isolation test, also try your RAM with one 4GB stick at a time, remove one and try with the other, and then switch them and try.
If in all these cases the system still crash then try a fresh install of windows, if it still crashes then you need to troubleshoot by isolation, by trying another graphics card.
Depending on your country if you are in Europe or the USA you will indeed find stores that offer you a 14 or 30 days return period even if you open your hardware's package, in such case just go down and buy any cheap AMD card and try it instead of the one you have and RMA the faulty part accordingly.
Before you do all these test you will not figure out what is going on and also we can not help you remotely when we do not know the results of these test .