There could be multiple reasons: not enough damping, not enough cross beams, missing rigid beams where needed... You can never tell what exactly is wrong until you compare the structure with a similar official vehicle.
So if it has all the beams it should have compared to that one, then it's probably not enough damping, try increasing it but not by too much.
Depends on the context. I think as long as it doesn't explode it's decent, I've seen values up to 300 in official vehicles on normal structures and even 1000 on damp helper beams.
