Pauper’s Drop problem

The game keep on jumping out to Finder whenever I finish Pauper’s Drop. WHY??? I use minimum settings at 1024 x 620. What is the problem? I mean if I have run so smoothly with the normal settings on my macbook pro from the starting story mode, why it stopped suddenly after finishing Pauper’s Drop? Please assist. Thanks~~~

$$Nitewalker$$ about BioShock 2