It might be errors in the map data provided to Garmin by Navteq. Garmin and Navteq both have places on their websites to provide feedback on any errors.
Having been in the GIS (Geographical Information Systems) field in a previous lifetime, you are sometimes limited by many factors when creating map data (like for Garmin). Data sources for products like this include everything from survey data from local municipalities to satellite photos. That's a massive amount of data to sift through, and it's possible that some data is either old or missed altogether. For years the map data companies thought US 287 from Bowie to Wichita Falls was a two lane road. It wasn't until the last version of City Navigator V7 from Garmin that this data point on a MAJOR route was fixed. Also, there is a section of US 75 in Oklahoma that is about 50 yards to the west of where it is located in the map data. Very frustrating to listen to Bitchin' Betty recalculate the route when I go that way.
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The Garmin I have, 276c, comes with accompanying software for my computer. When I get the erratic behavior, I take the unit over to the USB cord, plug it in, power things up, and re-download the maps and waypoints.
Don't know what causes the funky behavior, but reloading it solves it.