When games start off ok but gradually get slower, monitor your temperatures and voltage values for increases and fluctuations that could indicate a hardware problem.
Post back with 2
SensorsView screenshots, one from when you first switch on and another while playing the game when you notice it starting to lag.
Also, post your system specs, including the amps/volts info from the PSU label, and the layout of your case fans. Check to make sure all fans are spinning and the airflow from the front of the case to the back is unobstructed.
Which graphics driver do you have installed?