I could not reproduce the problem. (Start with 3rd person template, make a plank mesh from a bsp, set simulate physics on the static mesh, add a physics constraint component, lock all movement but twist motion. Result: Works perfectly smooth.)
Two ideas:
- Is your framerate stable and high enough?
- Do you have anything else that influences the plank except the player and the constraint? Are there maybe additional constraints or other objects with physics activated?
Also, what version are you using?