It could be a number of things, but let's take it one step at a time. I am assuming of course that your system meets the minimum system requirements for the game? Go to the game's website (or look on the box the game came in) to confirm.
Do the following in order, and if your problem is fixed at any point, you can skip the rest.
1. Go to the sim's website and download any updates they have.
2. Go to your video card maker's website, and download the latest drivers for it.
3. Go to your motherboard maker's website, and download the latest chipset drivers
4. Go to your motherboard maker's website and download the latest bios
I would also check if you are overheating. Try running with the case open and a fan blowing on your computer to see if that stops the problem.
You can also right click on my computer, select "manage" then "event viewer" and poke around in there to see what kind of error messages it is giving you. It could give some hints as to what the problem is.