You have a very nice laptop there actually. You should be able to run the game fine. With your concern about the processor speed that is its default operating speed. It can and will increase depending on processor load up to a Max Turbo Frequency of 3.1 GHz.
I suspect one of three things:
1. Your laptop becomes over heated. Try playing on a table top where it gives the laptop air or use a cooling pad. You can also try using a can of air to blow out the vent holes when it is off.
2. Your drivers are not up to date or your laptop has a feature called switchable graphics and is using your general card to play. (This is where you have two graphic cards in a laptop, when one is a gaming card and the other one is a general graphics card. The reason for this is to use the general graphics card when not playing games to not eat up battery power when unplugged)
3. You have malware/spyware that is eating up resources and causing your game to freeze.
But from the sounds of it, it is because your graphics card/CPU is starting to overheat and then dynamically down clocks / performs slower until it gets to hot and crashes. I hope I have answered your question and if there is anything else you need please let me know.
OS: Windows Vista
Processor: 2.4 GHz Dual Core
Memory: 2 GB RAM
Graphics: NVIDIA GeForce 9800 / ATI Radeon HD 3000 or greater
Hard Drive: 10 GB HD space
Sound: Direct-X Compatible
If I always appear prepared, it is because before entering on an undertaking, I have meditated for long and foreseen what may occur.
--Napoleon Bonaparte, 1831