I have that problem with Unreal Tournament on my laptop. It's caused by SpeedStep i.e. the CPU clock speed isn't constant - when the game loads, the CPU is at a lower speed so the game adjusts for that, but then during the game, the CPU speed steps up to maximum since you're now doing heavy processing, causing the game to run too fast because it doesn't compensate for it.

You can get round it by temporarily disabling the automatic speed changing before you start the game and forcing it at maximum speed even though it's not using all the power, such as going into Power Management and choosing a scheme other than Laptop.