While playing Assassin’s Creed Brotherhood I get serious choppiness

My pc specifications are following (Windows 7 Ultimate bit 64, Sapphire HD 5770 1 Gb, 4 Gb DDR2RAM, Gaming @ 1920×1080). While playing Assassin's Creed Brotherhood I get serious choppiness. While played first time not too much but some lagging. 1.5 GB ram idle when I checked Task Manager. Is it normal?
