Help regarding GTA San Andreas

I am trying to play GTA San Andreas on my MacBook but I keep getting problems. First, when I played it was beautiful with the highest resolution. My MacBook has the Intel GMA 950 and 512MB RAM with Core Duo 2.0Ghz. Then suddenly it was slowing down in a way. At the highest resolution CJ moved slowly and the whole framerate also slowed. So I set it to a lower setting. Now the framerate (meaning the allround movement) is better but when I pressed "Move Forward (W Key) for 2 seconds or more, CJ would start walking but wouldnt stop even if I let go. He would walk for sometime and then only would he stop. This makes it a problem when driving coz when I press W, the car goes on accelerating and hits people and the surrounding houses. Similarly when reversing. Confused So I guessed it was a problem with the Graphics so I downloaded and installed the latest Intel GMA950 drivers and restarted and it was playing beautifully at the highest resolution again. Hurray. I played like this for 1 and a half day. Very Happy
Then I ran into the mission of setting some house on fire with Molotov Chocktail mission and when I went inside to save the girl, the framerates slowed so I set it to a lower setting until I saved the girl and after finishing, I returned it to the original Highest setting but now it was like before and was slowing down Evil or Very Mad . Even when I set the lowest settings, CJ would run even after I had let go of the run button. And the driving was horrific. So now I cant play the game anymore Crying or Very sad . Please help. I love this game.