Grand Theft Auto 4 Stutter

[NIKEZ]

I'm running Grand Theft Auto 4 at around 40-70 fps, however I'm getting these annoying stutters like when I drive, it would feel like I'm having internet lag on single player; The game would sort of skip a frame. I looked around online and some people say it could be due to low space on the hard drive. My hard drive has about 15GB left in it, but I don't know, what do you folks think? All my drivers are up to date.

Specs:
Windows 7 Ultimate 64-Bit
Sapphire HD5870 1GB
AMD Phenom II X6 Black Edition
8GB DDR3