Is it in Riverview or Barnacle Bay?
If it's in either of these, bloated world cache files could be the issue. My game used to freeze a lot in these towns, and deleting the cache files always seemed to fix it. You'll find them in Documents/Electronic Arts/Sims 3/WorldCaches. Just delete everything inside that folder.
Even if it's not, try deleting the 4 cache files in your Sims folder and see if that helps before you try anything drastic (if you haven't already done so).