It's hard to say why this would be happening on your PS3. You can try rewinding within the affected episode, or restart the episode. Also, you could try reinstalling the base game (Episode 1). If you've customized your PS3, either with an aftermarket hard drive or more simply a custom background theme, this may be the issue.
While there may be moments in the game where the framerate dips, we never saw an issue like you've described on PS3.