Gta 4 question? Im really annoyed? help?

well its not gat 4 per-say, but its gta4, episodes from liberty city. I've played only the lost and damned so far, and my god does it lag! if i co on my chopper, irde a round and shoot a gun at someone, it lags. I just finished a mission where I had to chase an enemy biker gang and kill them and it lagged loads. why? I have a ps3 slim by the way. Is this normal? I hope its not my ps3 again because I had a slim, then replaced it because I thought it was the cause of the lag. I replaced it with this one. please help? Is this normal?