Hey,
Your PC is fully optimized for complete gaming, therefore I would suspect that there is nothing wrong with your PC in general. One thing though, how much RAM do you have? You didn't state on your spec sheet.
I'm guessing that it's the problem a lot of people have been having with Watch Dogs; it's a poorly optimized game. Meaning when developers released it, it tends to lag even with the greatest of systems. I know people with i7's and Titans in their systems and they still get lag spikes.
I believe one fix is to go to the main screen, and change one setting under "Display" and "Texture Resolution", changing the Texture Resolution for some people has helped with the overall lag from the game.
They really need to patch the game or something, it's outrageous.
Hope this helps,
Nobla_