collider sinking in the ground

I have experienced this problem in Unity3D 3.1. I have a car with a mesh collider (52 verts, 36 tris). sometimes the collider sink into the ground for some seconds and suddenly (like in the video) a great force push the collider out of the ground.

link to the video:

http://www.youtube.com/watch?v=nUgD6ah-zVY

The video is in slow motion, 1/10th of the realtime.

It seems that the problem lays on the fixed time step. with the value 0.002, the collision solver pushes them apart is a very strong way. with 0.02 (or 0.01) the collision solver seem more tolerant.

I tried to change min penetration for penalty and, while the behavior is a little better, it doesnt not solve the problem.

I tried to change bounce threshold, nothing. I tried to change solver iteration count, nothing.

this is the physic material for the ground: dynamic friction: 0.2 static friction: 0.4 bounciness 0 friction combine: average bounce combine: average

this is the physic material for the car dynamic friction: 0.6 static friction: 0.6 bounciness 0 friction combine: average bounce combine: average

PS: if i use a primitive collider (cube collider) it seems that the problem doesn't occur