Since it seemed to be working alright up till recently, you might try simply disabling some running programs to free up some of the processor's clock cycles.
Open a RUN command and type MSCONFIG, click on the "start-up" tab, click on "disable all", click "apply", click OK, restart and try the game out again and see if the problem is corrected..
You should also open up your computer and verify that the CPU fan is spinning and that the inside of the computer is virtually dust free....in other words, get yourself a can of compressed air and blow the dust out (if any) of the computer....focus on the heat sink of the processor....it's a really good dust collector