Custom vehicles (Especially for GTA IV) are usually the culprit for this. The high poly models you may be using are allocating too much of your video memory so it stops rendering the map. This is amplified during pursuits because not only is your game processing the AI, but also spawning in more high poly models as back up. Your best bet is to either remove the models or add them one by one and see how many your game can handle. Judging by your specs I'm assuming you don't have a dedicated GPU?
Note: This is personally what I have found over the years playing GTA IV, there may be many other culprits but for GTA IV with the map disappearing this is the most common issue I and many others have ran into.