RE: INSTABILITY ERROR ****WARNING INSTABILITY AT JOINT XYZ DIRECTION = FY/FZ

You can get general information on Instability and Zero stiffness in the FAQ section:

http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/2046.aspx#FAQ1

In your model, you need to use STAAD SPACE and use partial moment release as discussed in the attached document.