I have a Ronin S which sometime goes crazy, moving violently at random.

I use it with a Canon 5d Mk4, its perfectly balanced, autotuned etc. I'll be using it and it goes crazy, impacting the camera. The firmware is up to date.

I've seen similar results on youtube. Does anyone have any clues?

We apologize for your experience with Ronin-S, Canon 5D MK IV should be compatible with Ronin-S. Was the gimbal ever be collided?
If not, please make sure every knob in the gimbal has been locked tightly and there is no obstruction in the gimbal connection. Then ensure the lens support has been installed. If the issue persists, it is recommended to adjust the stiffness value in the APP to see if it works.