In case you weren't aware of it, SSAA renders the game at a higher resolution, then downsamples to your screen's actual resolution. If you're running 2xSSAA, that's 2560x1440. 4xSSAA? That's 4k load. It's not a problem with your hardware or bad optimization; a single 7970 just can't handle the game at that high a resolution with highest settings enabled, so your best bet it to just leave it off, and you might want to bring tessellation down to only high or medium to improve fps during fast action scenes (big firefights namely).