The first thing I'd do would be to pull the cable out of the speedo
cable tube and clean and grease it.  Every car I've had with a "nutz"
speedo has been cured by that.  When it's cold the cable binds in the
sheath and winds up and then releases making the needle jump
erratically.  As you drive it the friction warms up the cable inside
and softens the old, hard grease and it smooths out till it cools off.