Do this:
Download RivaTuner/Everest and monitor your temperature within the case and graphic's card temperature.
Get into the game, wait for the freezing/lags. Exit it and watch results.
(If they're still low and freezes, read next)
Go to GTA:SA Options and put everything on low, and view distance on low, and resolution too.
Wait for freezings, lags.
(If still freezes, read below again)
Go to (WindowsXP, dunno about Vista) X:\Documents and Settings\Admin\My Documents\GTA San Andreas User Files
(Where X: is the Harddrive, where GTA San Andreas was installed.)
And delete there gta_sa.set
Get back on the game (Don't change the graphic or anyother options yet) and wait for freezes
If the damn problem still continues......Re-install the game.
That's all....Maybe also downloading old drivers will help, but i don't really know....But that do only in the last place.