Here is a fix,
Click The Start Botton>Click Notepad>Copy and paste this:
-noprecache
-windowed
-novsync
-novblank
-viewdistance 0
-renderquality 0
-detailquality
-frameLimit 0
-texturequality 0
>Click File>Click Save As>Type: Command Line.txt > after this put it in your GTAIV Directory and download Process Tamer - Config>And Launch Process Tamer config and Launch GTAIV> And if it say Normal, It Should say Above Normal, just clikc the arrow and then Seclect Above Normal