There can be several causes of physics jitters. Make sure the TriMesh triangles are not too large in comparison to the other models like the car, it can cause jitters and even complete penetration.
Other things that can help with twitchy physics are changing the world/geom CFM values and adjusting the collision depth. Using multiple stepping per iteration can help improve the accuracy too (see the main loop in the 'CarDemo-HighSpeed' demo).
Using a rigid camera will also exaggerate the problem, this can be corrected by damping or smoothing the camera movement.
Other things that can help with twitchy physics are changing the world/geom CFM values and adjusting the collision depth. Using multiple stepping per iteration can help improve the accuracy too (see the main loop in the 'CarDemo-HighSpeed' demo).
Using a rigid camera will also exaggerate the problem, this can be corrected by damping or smoothing the camera movement.