1. Use disk defragmenter, just search it in the windows taskbar and you will find it. THIS WILL TAKE MANY HOURS, Leave your computer on overnight and let it do it, it will take a long time, and its worth it.
2. Download Gamebooster 3, and go into Tools- Defrag. Click on gta_sa.exe and defrag it. After do the same with mta_sa.exe. This will help a LOT, it will also increase you FPS.
If the problem persists, Re-install GTA, to get rid of any corrupt, or unneeded files.