Can you give me your full specs? I had a similar problem playing GTA IV EFLC on my old PC but as of now I have a new one with a GTX 680. Driver problem, maybe? I couldn't fix the problem with my old PC and then I thought to myself that its high time I biuld myself a High End Gaming PC.
I would go through your settings within the game and take a look at what you are currently using up. The game is good about telling you about how much ram you are taking up. Also, keep in mind you may be using a 1 gigabyte card and I know on max settings I get near 1.2 gigabytes of ram. Also agreed can you list your computer system specifications please.