It is very hard to tell, but I think the error is that something (one of your "point at" functions) is over-extending. Sorry, don't know how to describe this in English, but it is like the object has gone beyond the limit, and is trying to get back.

Mmm, that sounds like gibberish. Let me try again.

Somewhere one of the joints is going beyond the limit of either it's translate or rotate, and is still trying to point at something, but what it is trying to point at is now behind it.

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