It looks like you are binding your Combo to a datasource. If so, you have to keep DataMode to Normal, because AddItem mode is for unbound Combos that are populated using the AddItem method. I don't see you using the AddItem method so I assume you have a datasource.
Also make sure the DisplayMember matches the field from the datasource. Try this code for that line:
C1Combo1.DisplayMember = C1Combo1.Columns(1).DataField
Hope that helps
-Greg L