GTA San Andread lagging

[nilanko]

Here are my system specs:

CPU: Intel® Core™ i5-540M (2.66 GHz, 3 MB L3) NV
Motherboard: HM55 Intel® chipset
Memory: 4GB DDR3
HDD(s): Seagate 320GB 7200RPM 2.5" HDD with Active Protection System™
Graphics card(s): Discrete ATI® Mobility Radeon® HD 4570 with 512 MB DDR3
Sound card: Realtek HD Audio
Speakers: Integrated
Gaming peripherals: None
Optical drive(s): 12.7mm DVD-RAM LG
Fans: Dunno !!
Keyboard: Integrated
Mouse: Lenovo ThinkPad 3-button
Display: 14" HD (1366 x 768) LED-backlit (16:9), anti-glare display
Operating System: Genuine Windows 7 Professional Build 7600

I get an FPS of 35-110 FPS while playing GTA SA. Even though it seems as if the FPS is around 15. But note that the lag is not 'continuous'. It's periodical. It lags at every 2 seconds. One thing I have noticed is that during that small lag after every 2 seconds, the HHD indicator glows for the same time as that of the lag. It seems as if HHD becomes suddenly too busy and thus the game lags. What's the problem out there. Also the native resolution of my panel is 1366x768 but there isn't an option of the same in San Andreas but is there in Vice City. Can you help?

[vamsi_krishna]

Try Defragging your HDD. And updating drivers.