Today, I had my week 6 tutorial session and asked Alan how to fix it. after a long minutes going around the problem, the answer was simple, clusters can be really painful to control, as they often receive double transformation and move twice more than the mesh. So we figure it out another way to get the same effect. It consisted in creating a control based on joints similar to the eye rigging which later would receive a control and it would allow to move the strings.
A really interesting approach, that seemed really simple and effective. I have to say I'm pretty pleased with the result as it as the same effect than the previous wire tool approach, however it doesn't go crazy when you move the mesh around.
Also Alan suggested that maybe to create a dummy viol which would be carried around, one without any controls and the main viol which will be used for the playing scenes.