/fpslimit 100 or whatever lower that your comp may handle
Try other games and see if they work normaly.
Reinstall SA:MP
Reinstall GTA:SA
Reboot computer
Check computer memory in use
Check for overheating
Check for spywares and other things like that. They tend to slow down computers incredibly.
Clean inside your computer. It`s weird how dust can effect a computer performance.
Try buying a new hard drive and transfering your datas. Used hard drive can give bad performances.
Check your electrical input. If electricity is not constant, it may affect performance.
If all fail ... Eat a pancake, throw your computer in the street, ram it with your car and go buy a new one.