Do developers even test their games anymore? I mean, they were marketing the crap out of 'NO FPS CAP!!!!!1111oneeleven' and obviously never really play-tested this.
Gamers Nexus did an excellent investigation into issues surrounding this game and specific ways of 'making' horrendous stuttering happen - mostly it seemed to be hitting a frame rate cap (of about 140 fps) and also with CPUs with a lower amount of threads / cores. Definitely worth a watch if you're having issues with the game.
aah, the days when you could choose how speedy your game played... used to run gta (the original) at 640 x 480 for super fast chases, or 1600 x 1200 for slow mo gooranga! kills.