I had a nice opportunity to practice approaches in IMC several days ago here in S AZ. I practiced the KCGZ GPS RWY 5 approach multiple times and each time my GTN indicated LNAV+V minimums. However, on one of the approaches the CDI's (GI-106A) glide slope needle was flagged. There were no messages indicating any sort of degraded performance. Unit seems to work flawlessly otherwise. What would cause this? I can't find any explanation in the GTN manual.