Here's what you should do.
1- Add a text file called commandline.txt ( make sure it's not in caps ) in your GTA IV Directory.
2- Inside it, type in "-noprecache" ( without the " " )
3- Save it, and exit.
If that doesn't work for you, just remove the file.
And btw, if you are able to find a way that makes your computer process the game as software instead of hardware, try doing that. I did, and it gave me a huge increase in performance.