[Whizion]

If it lags when you go in to the car then i had this problem to.

Do this:

Go here: C:/Program Files/Rockstar Games/GTA San Andreas/audio/streams (or where you installed GTA SA)

Delete this files:CH CO CR DS HC MH MR NJ RE RG TK

Don't delete this: AA ADVERTS AMBIENCE BEATS CUTSCENE

Start the game, it shouldn't lag anymore.