How come my computer freezes in a black screen on Sims 3?

I use Windows 7, laptop, 4 GB RAM, and while I edit a 60 X 60 lot on edit town mode, it freezes or lags, and then it returns with computer brightness all the way up then I can lower it. Now it just freezes and I have to take out the battery just to force the computer off. What could the problem be? By the way, the house is a copy of the Landgraab estate between the Alto & Landgraab manors. Could the game not handle 3 mansions side by side? Or what else could be wrong?