Firstly, before you do this, have you cleared your DCBackup and the like to speed up your game? Secondly, are you using an old save file? Perhaps it would help to move your sims (and buildings?) out and put them in a new save file. Relationships with townies will be lost, I hope that isn't too much of a problem, and it shouldn't be, considering re-installation will do the same. Thirdly, your operating system may be a little worn out itself, either through time or your wonky sims.
Reinstalling will get rid of all the stuff you've downloaded, but if it's store stuff then it's pretty easy to re-download. Your download history is available on the official Sims 3 site and all stuff can be re-added with any extra charge. If it is from elsewhere, then you should check a note of the addresses you got them at in a word document. Your Sims 3 account would be a good place to back up buildings and sims that you don't want to lose.
Hope this helps!