I contacted to blizzard support and they told to try differend things and none of em did work. So eventually they said that the card might be too old.
Can HD 5870 be outdated to wow?
So what can cause in graphic card to go 0% activity for being too heavy to handle? Is it running out of memory or what? I tried to lower quality settings but didn't have any major difference in FPS.
I also tried to overclock my card but when I added even a bit more clock to it, I started to see gliching on my screens. Might there be something physically wrong at my card?
Can motherboard affect graphic card performance like that?
Sorry for I'm not so familiar with this things but thank you for all replies. I hope to get this solved soon.