I was playing on my Radeon HD7870 when out of nowhere my game/PC froze with gray visual bars on the whole screen. I had to manually turn it off and when I tried to turn it on, I got a black screen after Windows 8 loading logo (circle). And it happens everytime now. Almost, because 1 on 10 resets it will sometimes show this recovery screen: Just a moment...

I managed to log in and get into my desktop by selecting continue to win 8.1 option. All was working fine (programs, browser) till I started the game again and maybe after 3 minutes same scenario.
From that moment continue to win 8.1 option is no longer working it just stays on black screen because I have now artifacts even in recovery screen and even in safe mode. Some small squares or kind of visual deformations not the whole screen with bars.

I think it looks like GPU but several times people were telling me "it's your GPU dying" when I had e.g. monitor cable problems so I'd like to check everything what's possible. I'll be very gratefull for any type of help.

Win 8.1 x64
Radeon HD7870
i5 3450s
Chieftec ctg-500-80p

Thanks in advance.