savvannah_007 That does help: it means the game is probably not running out of RAM. You can check just before you save, and as long as the value is under 3.4 GB, that shouldn't be the cause of the Error 12 message. (Saving takes an extra ~300 MB, and some of the game's 4 GB capacity is soaked up by overhead.) But Error 12 is an out of resources condition, not just RAM.
Still, make sure your system isn't using close to its overall capacity. 50% in use is fine, but over 90 would mean that temporary spike from some other source could eat up enough of the reserve to put Sims 3 in the danger zone when you try to save.
What graphics settings are you using, particularly for high-detail lots? If it's more than 2, try turning it down during your next play session and see whether that helps. Water is the next-most-demanding setting, and although I don't think it should be a major contributor to this particular issue, you can try turning it down to "mirrors only" as well.
If this doesn't help—RAM never reaches the danger zone and turning down the two graphics settings makes no difference—try playing with your computer offline. You can sign into the EA App and put it in offline mode, then disable wifi and/or disconnect the ethernet cable before pressing Play.
If that doesn't help either, try a new save in Sunset Valley, for comparison's sake.