Wheel Colliders bouncing/misbehaving when you tweak friction parameters. If you increase the Stiffness or Extremum Value in Forward Friction settings to make vehicle accelerate realistically with less tire slip, the Wheel Collider component starts bouncing/misbehaving.

To reproduce:

1. Open project.
2. Go to "_Scenes" folder and open "main" scene.
3. Enter Play Mode.
4. Observe the "Test Vehicle 2" game object.

Expected results: Vehicle slips the tires at first, and after a while achieve grip with the ground. Smooth ride throughout the test.
Actual results: After a second of acceleration the vehicle starts to bounce. Gaining and loosing grip very frequently (looks like this is happening, not sure).
(Video: "Example1.webm")

Note: The "Test Vehicle 1", the vehicle next to the bouncing vehicle is identical except for small Stiffness parameter.
The bouncing vehicle has a Stiffness of 3.5, but you start to see the effect starting from Stiffness = 2;

Additional information: You can observe similar results when increasing Extremum Value in Forward Friction.

The problem might be that the wheels starts to spin a little bit to fast and decreasing the torque is the way to go, BUT if you decrease torque the acceleration is unrealistic, very slow.

Reproduces with: 5.3.6f1; 5.4.4p3; 5.5.2p2; 5.5.2p3; 5.6.0f1 spin2; 2017a4;