Try turnign off your AA. What resolution are you using? What is your nVidia card? MSI geforce 1 gb ddr3 only tells me that you have a nVidia card with 1gb of Vram. So which card is it?
GT 430, GTS 450, GTX 460? What is the brand of your PSU? When it lags and fps drop then they go at what speed for the rest of the game? I know I ask to many questions. Get back to us with the answers please. Good luck.
My card is a MSi Gtx 460 1GDDR5, not sure what my psu is. I mean I have everything on high with v sync. I play the game smooth, but at some parts it bounces back from 30-60 fps. Then it goes back to 60.
That kinda sound normal. When I play Shogun 2 total war. When I am on the campain map I get around 25 to 30 fps and in the battle screen I get more than 70 fps. The part that lags is what is a problem.
Check and make sure your power managment is set for preformance and you are not using a power saving turned on. You have a good rig set up so in less it is in the settings you shouldn't have that problem.
Have you tried turning off your v sync? In the nvidia contro panel have you gone into there and mess around with the manage 3D settings? It might help a little but you will have to do it trial by error.
It is going to take a little time so keeps notes about what you did. Good luck. Cheers