Hi Talon,
I'm sorry of my late reply! I don't connect very often to the forum and I did not see your answer until yesterday!
It really seems to be a "mechanical" shaking. It does not seem to be linked to her mood.
She is indeed in old stage.
This behaviour is sometimes present and sometimes absent. Lately, she hasn't shaken (which is good news!) But therefore, I don't have a video of her behaviour. As soon as I have one, I will post it here or send you a private message.
In any case, thank you very much for your helpfulness

and really sorry for my late answer!