1. control that the polar you are using is correct (attention: do not place too much confidence in the polar data that can be found in the web, in the forums or that other pilots declame! We noticed that in many cases Polars are too much optimistic). Maybe your Polar is too optimistic or too negative;

  2. control that the Altitude is correct (if you do not use the auto calibration from GPS);

  3. control that a terrain map is active and properly uploaded in the terrain folder for your flight area;

  4. control that the terrain map file you are using is not corrupted;

  5. control the 3D-fix is active and stable.