I'm not an expert, but the first thing I would check is whether the cars are working. When choosing a car flick through them all and see if the car appears in the window and spins around. If no car appears, then it will crash. If all bar 1 or 2 cars appear, it's those cars that are the problem.
If this isn't the problem you'll need to set up and run a trace file (use the forum search function to find how to do this, if you don't already know), to locate the problem. This will also tell the guys who know what they are doing here, what your problem is.