Maybe you tired this but try reinstalling your GTA into a different folder in different virutal hard-drive. Totally delete GTA SA, then run a coockie / trojan tracker / cleaner, Run your anti-virus and delete every threat. Check your desktop, if it's full with useless junk, delete it because it slows your computer. Run your task manager - go to processes and check for programs which takes much RAM memory and aren't used. Check your hard-drives, avoid your main virtual drive (The one which contains Windows) from being full and filled with downloaded, videos, movies. Try GTA after that, if doesn't work, I guess the next step is to format your computer, if it doesn't work even after formating your computer then my best guess is that you'll have to buy a new computer.
P.S: I know it seems much to do but that is what i'm usually checking and doing if my computer starts to run slow.