That seems (to me) to be a fairly common problem across all the brands that I have used. Garmin and built-in's in a Toyota and a Ford. They all seem to do strange things at times.
Looking at a map each morning to get roughly familiar with the route helps. So does watching the map on the screen as you approach towns. Some of the craziness is easy to spot (and avoid) if you watch the screen instead of blindly following the voice prompts.
Bottom line: A GPS should be used ONLY as an aid. YOU are still responsible for not getting yourself into trouble.