I have the same cpu in my spare rig, make sure in your bios you have hyperthreading on, by default it is off, enable enhanced turbo boost and high performance computing mode as well, should be an option in there. make sure you have your power profile set to performance as well.
Mine will stay at 3.15 ghz when gaming , the stock cpu heatsink is garbage atleast the one in my hp z400, check to make sure the cpu isn't overheating.
My games run great with my 7970 in there, plays just as well as my 9370 @ 4.8 ghz.
leave vysnc off, its most likely the 750ti depending on the game