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.