@Simstar3 These steps may not apply to you but here are a couple of other suggestions, followed by some questions.
1. Disable any custom content if you have any. Badly designed CC could conceivably cause this problem, though it's unlikely.
2. Delete the game caches if you are comfortable manipulating files on your computer. (see below)
3. As Pam says, you can try reinstalling.
One solution you can try before reinstalling completely (remember to deinstall first, especially if you have downloaded versions of any of the EP's or the game itself) is to delete the game caches. If you are comfortable messing about in folders and with files on your computer, you can find instructions here. Note that if you have a mac, do not delete the WorldCache.
[Link removed. - Pam]-----------------------
Assuming you're running a windows machine, what is your graphics card? Processor? RAM? Free hard drive space?
Here are instructions for how to find out your graphics card:
http://www.ehow.com/how_4916955_out-graphics-card-windows.htmlIf you right click on "Computer" in the Start menu, and select "Properties" you should be able to find your processor and Installed Memory (RAM) under "System". (Can't recall if this works in Windows XP but does in Vista and 7).
Are you running other programs (e.g. anti-virus, browser) while The Sims is running?
@John some good points. One tip that you're probably aware of if you are installing fans inside your PC -- be careful if you're getting near the edge of your system's power envelope. I've had annoying problems with TS3 and power use.