Multiple rigidbody coming to rest on top of rigidbody causing strange forces.

If more than one rigidbody rests on top of another rigidbody I am seeing odd forces being applied. I have rigidbodies that collect on the player in my game and weigh very little but cause odd behavior. I have tried many things including changing distance scale, weight range, physics material...

Attached is a very simple project for example. Open the scene. There will be a "Top" attached to "Base" with a spring joint. There are two other "Bits" that are yellow. One rests on the "Top". The other yellow cube is above the "Top" when you start and is set to kinematic.

Run the scene and then disable Kinematic on Bit2. It will drop onto the top and initially cause a small downwards force as expected. However, after that the whole top raises up higher than it ever was! Now disable one of the bits and note how the top drops down. This is opposite to what should happen. A single bit will behave fine but when more than one are resting on top it causes odd behavior.

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I have tried testing other things like acceleration on a simple box and the problem is same. It is nothing to do with the spring joint. I am using Unity 4.6 and have tried previous versions. Problem does not appear in Unity 5 but I need to use version 4 at this time.

Could someone please test this project and let me know if I am going bonkers? Seems like a Unity bug but this is such a basic function.

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