It was the thread. I bought a replacement screw of ebay. The knob and handle was in rather bad condition with deep pitting. However, the thread was covered with heavy grease and cleaned up quite well.
The half nut does not jump away of the screw in any position. A great improvement. I guess the original screw must have had very busy life and spent most if its life in the first 4″.
The vice now works as it should. Esthetically though it us far from what I would be happy with. Need to use some more elbow grease on polishing the handle and the knob.
Just wondering, what do you guys use on the thread? A grease or an oil?