Hi Eminem,
What you are experiencing is a process called Artifacting.
This happens when your graphics is getting a bit too hot! You can do any of the following to help prevent this.
The first one to try is to find out if you have any newer graphics drivers, this might help with the problem and fix it.
The Second is not the best fix but it should solve it. When it starts doing it, you should wait about 10 minutes ( have a coffee or browse the forums!) until the graphics card cools down.
If you are using a machine that is less that two years old, you may find that the drivers should help. If not, try setting your graphics settings in GTA SA, to something a bit lower (less Anti Aliasing For Example)
Happy Gaming!
Regards,
DellStorm