If you have any suggestions how to improve the physics please say :)
I reckon you use the Vehicle class.
One thing you should do is verify that your model rotations and transformations are correct. If you're using Blender, be sure to Export your models using the "XNA strict options". The coordinate systems in Blender are different from XNA. in the top-view (Num7) of your model in Blender, you should see the vehicle from behind and the bottom of the vehicle's chassis should be positioned on the x-axis. This should avoid cars being rotated and stuck in the ground.
Another thing you could do is experiment with several values when setting up your vehicles, e.g. mass, rigidness and the wheels' suspension. The heavier the car, the more stable it gets. I'd suggest using Digital Rune's provided VehicleSample and experiment with different values for each car you have. Once you get satisfying results, you store these values in an XML-file, that you can use in your game for the initialization.
BTW: I must accompany my previous posters: really impressive work so far!