This discussion may contain spoilers!
I'm using Mac OS X (10.6.6) and there's these two things:
1)When launching the game, the music starts playing but the screen shows random stuff, apparently from a buffer. There are pieces of windows I just closed and my desktop etc. 2-3 seconds later it shows the Delorean in the car park. I'm guessing I'm either somehow missing a scene before that or the first few seconds of that one.
2)Whenever bright light is supposed to appear on screen (examples, with some spoilers: The Delorean travels while Marty shoots the video, or near the end when the camera pans to the street and the car with Kid and the Doc appears) the screen freezes for a few seconds and then "jumps" to the correct frame where the light is still on but no more slowdown happens.
Both things happen even if I lower the graphic detail to the lowest setting. I'm using a recent imac (bought on November).