Is this through Steam? If so try make a shortcut and launch game from LaunchGTAIV.exe instead of launching it via Steam. This helped performance for me more so than using commands.
This game doesn't run well at high fps i noticed and animated cut scenes being recorded at 30fps can have a choppy look when fps was too high so i capped my frame rate. 30fps was too low for me during game play but 45fps was pretty good and cut scenes played normal. Try cap fps the same in your AMD software.
Did notice however,, the game's performance did improve as i progressed further.