I had a similar problem with another game with Steam. The problem was caused by the web-shop inside Steam. The frontpage of the Shop scrolls automatically to show the recent games and offers. THAT causes the shuttering for me. To avoid this just click the shop and click, for example, Comunity activity. That page shows the statistics of the players in Steam and doesn't have any type of scrolling. Then launch the game and check if that was the problem.