GTA: San Andreas Problem

[KholdShock]

After playing fine with no lag to speak of for about 20-30 minutes the game starts to show signs of lag as if I was in a congested area with many things going on, and then goes back to normal. It goes back and forth like this for about 10-15 more minutes, and gradually this lag get worse, comes to a crawl(<1 Frame per sec), and locks up to where I have to hit the reset button. I have tried turning my settings down to low, reinstalling, and updating my graphic card drivers from Nvidia.

My machine is more than adequate: P4 2.6Ghz, 1GB of ram, 128MB GeForce 4 TI 4800, 120GB SE West Digi HDD.
Any Suggestions? TIA