First of all check if anything is Thermal Throttling (i.e. CPU or GPU). Also check if too many programs are running in the background via Task Manager as some programs can use fair amount of CPU and/or RAM which slows things down. GTA IV is a bit poorly optimized game. What resolution you're playing the game at? If its 1080p then your setup is powerful enough. If its 1440p then dial back some of the setting such as Anti-Aliasing, Ambient Occlusion and/or Texture Quality.