Car made of boxes teleports on hard collision

I am trying to develop a simple car racing game. The cars comprise of 2 solid cuboids (one engine and one for body) and 4 cylinders. All the objects are connected to each other by fixed joints. The engine is the part that moves and carries the rest with it.

Sometimes on hard/repetitive collision with another car or a wall the car parts behaves crazy for a while and the assemble at another point in the scene.

Any idea why this is happening?