Shaking sounds like a flybar. Get a new one and see. If it keeps doing it then it could be the main shaft is bent. The slightest bend in those causes all kinds of problems. They are around $8 each and it is good to have an extra one on hand. The inner shaft also bends easy, usually at the top where the head attaches. Those are cheaper as the gear is not attached to the bottom like on the main shaft.