Hmmm. Okay... slightly crazy idea, but let's see if it works.
Rename your "The Sims 3" folder under Documents\Electronic Arts. Just call it something like "The Sims 3 (Backup)". Launch your game so it has to generate new files.
Once you've done that, set the game up how you like in options and see if it plays alright on the normal worlds. If it's all working ok, copy the downloaded world and your save (only these) from your old folder to the new. See if that does the job.
If the game plays fine before copying the save over, but doesn't AFTER copying it... you may have a corrupt save. If the save file continues to play fine, you probably just had a corrupt cache file or something.