I'm experiencing this stutter and lag too. To be precise, while Episode 1 had sound issues and lag initially, it smoothed out. Episode 2 is virtually unplayable though, it lags every 2 or 3 seconds, and the sound is constantly glitching and repeats phrases while skipping dialogue.
Though I'd love to try and do what Kahlusha said they did, I can't say I find wiping my PS3 and going through backing up files or replacing the consoles hard drive, for a game that I feel should work smooth in the first place, to be a solution. I hope that doesn't sound rude... but I find it weird that a console game of all things isn't working properly on a console.
I'm curious if the 360 version has similar issues? I have a 360, PS3, and gaming PC, and somehow picked the wrong version of this game apparently.