Someone's thousandth post, about Sims 3 crashing for no apparent reason... seems appropriate. Anyway.
Does the game always crash at the same in-game time, or does it vary? If it varies, does a brand new save, not new-ish but one you create for testing purposes, also crash? There are so many reasons the game can crash, the first step is narrowing things down.
If a new save crashes, please test another new save, this time in a clean user data folder. Move the entire Sims 3 folder out of Documents\Electronic Arts and onto your desktop, launch the game to create a clean folder, start a new save, and see how long you can play.
If the new save crashes in the clean folder, please run a dxdiag, upload it somewhere, and link it here. Hit Windows key-R, enter "dxdiag" without quotes, wait for the scan to finish, click "Save all information," and save the .txt file to your desktop. From there, you can post it on a filesharing site (e.g. the free version of pastebin.com) and link it here.