I did some more reading on the problem, since one of the symptoms of this issue is the laggy game play during/after travelling (which I have been experiencing lately). There is some very detailed information on another website, and someone has developed a mod that may help with this problem in several ways. Personally I prefer to steer clear of mods if they are too much work (I'm tired of spending hours trying to get something to work that EA hasn't fixed), and the stance in this forum is that they are not reccommended and can't be discussed. Other than making multiple back-ups, they also suggested turning story progression off. I don't know if it's possible to do manually, but you could try deleting excess cars around the neighbourhood, and turning off stereos in other sims' homes, as these are known to be just a couple of the causes of this problem.