The cplane should not change by itself, but you may be accidentally hitting a hotkey combination that changes it? The default keyboard file uses CTRL+number keys to set the cplane, so if you hit CTRL+5 it will change to cplane 5. But how can anybody work in a 3D model with CP=DV? The only time I ever use CP=DV is if I'm adding text oriented to a particular isometric display view.