Batman Arkham Asylum is very laggy but I have met all requirements, please help!

[Red Haze]

Hello everyone, I recently bought Batman Arkham Asylum GOTY from steam. At the start the game played alright with a little lag occasionally but it never really ran smoothly. After a few days of playing, the game is now extremely laggy, and in some levels its unplayable. The part I'm on now is so laggy the game actually skips a little in the cinematics, its just not enjoyable like this :-/

Here are my PC specs:

OS: Windows 7 Ultimate 64-bit
CPU: Intel Core i5 750 2.67GHz
Graphics: Nvidia GeForce GTX 560 4050MB
Display Mode: 1280x1024
RAM: 8GB
Game Settings:
Details Level: High
Hardware Accelerated PhysX: Normal

"Can You Run It" results:


My PC should be more than enough to run this game in High Detail, but it still lags. If there is any information that would be useful that I have missed out here, please let me know and I will add it as soon as possible

Hope you guys can help