Well I have a GTX 760 just like you. Also like you I had performance issues. But this was resolved when I upgraded my processor from an AMD FX 6100 to a AMD FX 8350! But it looks like you don't have that sort of problem. Have you updated all of your drivers? Have you defragmented your hard drive? Where do you keep your games and Steam stored? On your SSD or HDD? Any virus protection software? Just go through the list of things that could potentially be wrong.