That basemap is maybe too big. You can try using the small Garmin basemap recommended for GMXT, it's old [2007] but may overcome the 'choppiness'. The link's in [Only registered and activated users can see links. ], or you could remove DEM from V3 or V5 basemap using GMapTool. The other possible problem is how you 'extracted' USA & Hawaii from the full NA image. Did you use GMapTool to take parts from the full NA map image? If you only want a part of a map it's better to use BaseCamp/MapInstall or even MapSource [getting a bit old though] to take the parts you want from a gmap folder and that way create a correctly compiled image.