I have the I7 920 and a 970, I am pretty sure the CPU is the bottleneck, same w my situation. Fortnite wont even run at a stable 70-80fps at 1600x1080 for me. I get around 90fps in R6S, medium settings. Thats why im currently looking for a cpu upgrade, 6600k or 6700k rn, because i have a brand new z170 mobo laying around for years.