I'm only a mechanic of the shadetree variety so I don't know the exact physical properties of your particular hardware. That said, I'm going to suppose your stem has parts similar to anything I've ever worked on - cups, cones and caged ball bearings. The cups are probably pressed into the headpiece tube and are the same, both top and bottom. Remove the cups, clean all the parts and apply fresh grease, install the top one on the bottom and the bottom one on top. As long as nothing is cracked or broken this method is usually adequate to get rid of the "choppy" feeling when steering.