Play the game windowed and it fixes the overlay issues which cause the lag.
Yes I agree, playing in windowed mode could solve a few issues in the way the game is rendered.
Using something like the free Defraggler to defrag your game folder quickly would help stuttering. Ideally, do all your drives. Check out TweakGuides Tweak Companion as that covers alot of areas in more detail if you need further help from an expert as its free.
Also, have you tried disabling typical gamer apps like Xfire, Fraps, antivirus. Try the same with other apps that aren't game related in your taskbar and check that applications are running on a schedule during games.
On a side note; could someone tell me why this game would run this game at a different refresh rate than what I have it set to on my desktop? The game only allows 59Hz refresh @1680x1050 but other games/desktop runs fine at allowing selection of 60Hz.