Steps to reproduce:

1. Open attached project
2. Open scene "wheelColliderProblem"
3. Notice that game object "vehicle" has "Rigidbody" with "Mass" set to "0.04"
4. Notice that game object "payload" has "Rigidbody" with "Mass" set to "1" and has "Configurable Joint" connected to "vehicle"
5. Run scene
6. Notice that "vehicle" keeps bouncing. After waiting some more time "vehicle" flips over.

Note: If we set "Mass" of "payload" to "0.02" then this problem doesn't occur. If we remove "Configurable Joint" from "payload" and add it to "vehicle" this problem will still occur.