[Scenario]

Make sure your running gta_sa.exe on a single core (not dual cores). You may wish to delete the gta_set.set file in your GTA San Andreas User Files folder (usually located in your documents folder). Also make sure the drivers for the video card have been installed properly and are up-to-date.

You may want to try playing another game, such as GTA: Vice City just to test if it may (or may not) be the card.